On February 24, 2021, ICON public limited company (which is referred to as ICON), ICON US Holdings Inc., a wholly owned subsidiary of ICON (which is referred to as US HoldCo), Indigo Merger Sub, Inc., a wholly owned subsidiary of ICON and US HoldCo (which is referred to as Merger Sub), and PRA Health Sciences, Inc. (which is referred to as PRA) entered into an Agreement and Plan of Merger (which, as it may be amended from time to time, is referred to as the merger agreement) that provides for the acquisition of PRA by ICON. Pursuant to the terms of the merger agreement, Merger Sub will merge with and into PRA, which is referred to as the merger, with PRA surviving as a wholly owned subsidiary of ICON and US HoldCo.

Upon successful completion of the merger, each issued and outstanding share of PRA common stock (other than excluded shares and dissenting shares as described in the merger agreement) will be converted automatically into the right to receive (i) 0.4125 of one ICON ordinary share, which number is referred to as the exchange ratio, and (ii) $80.00 in cash, without interest. The exchange ratio is fixed and will not be adjusted for changes in the market price of either ICON ordinary shares or PRA common stock prior to completion of the merger. Because the exchange ratio is fixed, the market value of the merger consideration to PRA stockholders will fluctuate with the market price of ICON ordinary shares and will not be known at the time that ICON shareholders vote on the ICON share issuance proposal and PRA stockholders vote on the PRA merger agreement proposal. Based on the closing price of ICON ordinary shares of $208.62 on Nasdaq on February 23, 2021, the last full trading day before the public announcement of the merger agreement, the implied value of the merger consideration to PRA stockholders was approximately $166.06 per share of PRA common stock, and after applying the exchange ratio of 0.4125, ICON and PRA estimated that, immediately following completion of the merger, former PRA stockholders will hold, in the aggregate, approximately 34% of the issued and outstanding shares of ICON, and ICON shareholders as of immediately prior to the completion of the merger will hold, in the aggregate, approximately 66% of the issued and outstanding ordinary shares of ICON. On April 26, 2021, the latest practicable trading day before the date of this joint proxy statement/prospectus, the closing price of ICON ordinary shares on Nasdaq was $217.88 per share, resulting in an implied value of the merger consideration to PRA stockholders of $169.88 per share of PRA common stock. ICON ordinary shares are traded on the Nasdaq Global Select Market (which is referred to as Nasdaq) under the symbol “ICLR” and PRA common stock is traded on Nasdaq under the symbol “PRAH.” The ICON ordinary shares issued in connection with the merger will be listed on Nasdaq. We encourage you to obtain current quotes for the ICON ordinary shares and the PRA common stock.

ICON will hold an extraordinary general meeting of its shareholders in connection with the share issuance comprising the share consideration in the merger, which is referred to as the ICON EGM. At the ICON EGM, ICON’s shareholders will be asked to consider and vote on (1) the proposal to approve the issuance of ICON ordinary shares to PRA stockholders pursuant to the merger agreement, which is referred to as the ICON share issuance proposal, and (2) the proposal to adjourn the ICON EGM to solicit additional proxies if there are not sufficient votes to approve the ICON share issuance proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to ICON shareholders. The ICON board of directors unanimously recommends that ICON shareholders vote “FOR” each of the proposals to be considered at the ICON EGM.

PRA will hold a special meeting of its stockholders in connection with the proposed merger, which is referred to as the PRA stockholder meeting. At the PRA stockholder meeting, PRA stockholders will be asked to consider and vote on (1) the proposal to adopt the merger agreement, which is referred to as the PRA merger agreement proposal, (2) the proposal to approve, on a non-binding advisory basis, specific compensatory arrangements between PRA and its named executive officers relating to the merger and (3) the proposal to adjourn the PRA stockholder meeting to solicit additional proxies if there are not sufficient votes to approve the PRA merger agreement proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to PRA stockholders. The PRA board of directors unanimously recommends that PRA stockholders vote “FOR” each of the proposals to be considered at the PRA stockholder meeting.

We cannot complete the merger unless the PRA stockholders approve the PRA merger agreement proposal and the ICON shareholders approve the ICON share issuance proposal. Your vote on these matters is very important, regardless of the number of shares you own. Whether or not you plan to attend the ICON EGM or the PRA stockholder meeting, as applicable, please promptly mark, sign and date the applicable accompanying proxy card and return it in the enclosed envelope or authorize the individuals named on your proxy card to vote your shares by calling the toll-free telephone number or by using the Internet as described in the instructions included with your proxy card.

The accompanying joint proxy statement/prospectus provides you with important information about the merger agreement, the merger and the other transactions contemplated by the merger agreement, the ICON EGM, each of the ICON proposals, the PRA stockholder meeting and each of the PRA proposals. We encourage you to read the entire document carefully, in particular the “Risk Factors” section beginning on page 34 for a discussion of risks related to the merger and the combined company after the merger.

We look forward to the successful completion of the merger.

Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of the merger or the ICON share issuance in connection with the merger or determined if this joint proxy statement/prospectus is accurate or complete. Any representation to the contrary is a criminal offense.

This joint proxy statement/prospectus is not intended to be and is not a prospectus for the purposes of the Companies Act of 2014 of Ireland, the European Union (Prospectus) Regulations 2019 of Ireland (as amended) or the Prospectus Rules issued by the Central Bank of Ireland, and the Central Bank of Ireland has not approved this joint proxy statement/prospectus.

This joint proxy statement/prospectus is dated April 27, 2021 and is first being mailed to ICON shareholders and PRA stockholders on or about April 29, 2021.

Notice is hereby given that ICON plc, which is referred to as ICON, will hold an extraordinary general meeting of its shareholders, which is referred to as the ICON EGM. The ICON EGM will be held on Tuesday, June 15, 2021, beginning at 3:30 p.m., Dublin time, at ICON’s global headquarters in South County Business Park, Leopardstown, Dublin 18, Ireland.

The ICON EGM is for the purpose of considering and, if thought fit, passing the following resolutions as ordinary resolutions:

Ordinary Resolution 1: The ICON share issuance proposal

“That, subject to applicable rules and listing standards of Nasdaq, the Directors of ICON be and they are hereby generally and unconditionally authorized pursuant to section 1021 of the Companies Act 2014 to exercise all the powers of ICON to allot relevant securities (within the meaning of section 1021 of the Companies Act 2014) as contemplated by the Agreement and Plan of Merger, dated as of February 24, 2021 (as it may be amended from time to time), which is referred to as the merger agreement, by and among ICON, ICON US Holdings Inc., Indigo Merger Sub, Inc. and PRA Health Sciences, Inc., up to an aggregate nominal amount of ICON ordinary shares necessary for purposes of satisfying the aggregate issuance of ICON ordinary shares in connection with and pursuant to the merger agreement, provided that such authority shall (a) expire on December 31, 2022, or such later date as may be determined by the ICON board of directors from time to time (provided that under the Companies Act, such later date cannot be more than 5 years after the date on which this resolution is passed), and (b) be without prejudice and in addition to the authority under the said Section 1021 previously granted to the ICON board of directors pursuant to an ordinary resolution passed at the annual general meeting of ICON held on July 21, 2020.”

Ordinary Resolution 2: The ICON adjournment proposal

“That the Chairperson of the meeting be and is hereby authorized to adjourn the meeting to another time and place if, in the discretion of the Chairperson, it is necessary or appropriate to, among other things, solicit additional proxies if there are insufficient votes received by way of proxy, at the time of the meeting to approve the ICON share issuance proposal.”

Resolution 1 is referred to as the ICON share issuance proposal and Resolution 2 is referred to as the ICON adjournment proposal. Resolution 1 is required to be passed as a condition to the completion of the merger agreement (as defined in Resolution 1), and Resolution 2 is not required to be passed as a condition to the completion of the merger agreement. The resolutions may be voted on in such order as is determined by the Chairperson of the ICON EGM.

The accompanying joint proxy statement/prospectus, including the merger agreement attached thereto as Annex A, contains further information with respect to these matters.

Only holders of record of ICON ordinary shares at the close of business on April 26, 2021 are entitled to notice of and to vote at the ICON EGM and any adjournments or postponements thereof.

The ICON board of directors has unanimously approved and declared advisable the merger agreement and the transactions contemplated by the merger agreement on the terms and subject to the conditions set forth in the merger agreement. The ICON board of directors unanimously recommends that ICON shareholders vote “FOR” the ICON share issuance proposal and “FOR” the ICON adjournment proposal.

Your vote is very important, regardless of the number of ICON ordinary shares you own. We cannot complete the transactions contemplated by the merger agreement without the approval of the ICON share issuance proposal. Assuming a quorum is present, the approval of the ICON share issuance proposal requires the affirmative vote of a majority of votes cast on the proposal.

We urge you to please promptly mark, sign and date the accompanying proxy and return it in the enclosed envelope or authorize the individuals named on the proxy card to vote your shares by calling the toll-free telephone number or by using the Internet as described in the instructions included with the proxy card. If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.

If you have any questions about the merger, please contact ICON at 1-888-381-7923 or write to ICON plc, Attention: Investor Relations, South County Business Park, Leopardstown, Dublin 18, Ireland, or via email at IR@iconplc.com

If you have any questions about how to vote or direct a vote in respect of your ICON ordinary shares, you may contact our proxy solicitor, Georgeson LLC, via phone at 1-866-295-4321 (toll-free within the United States) or at 1-781-575-2137 (outside the United States), or via email at ICON@georgeson.com.

Your vote is important. ICON shareholders are requested to complete, date, sign and return the enclosed proxy in the envelope provided, or to submit their votes electronically through the Internet or by telephone.

In light of public health concerns related to COVID-19, ICON would like to emphasize that it considers the health of its shareholders, employees, attendees and other stakeholders a top priority and in this context is closely monitoring the evolving COVID-19 situation as regards attendance at the ICON EGM. Shareholders are referred to the section entitled “The ICON Extraordinary General Meeting” which sets out further detail on the public health situation in Ireland and current restrictions around in person attendance at the ICON offices. ICON therefore strongly encourages voting via mail, telephone and Internet over in person attendance at the ICON EGM.

Notice is hereby given that PRA Health Sciences, Inc., which is referred to as PRA, will hold a virtual, audio only special meeting of its stockholders, which is referred to as the PRA stockholder meeting, at www.virtualshareholdermeeting.com/PRAH2021SM, on Tuesday, June 15, 2021, beginning at 10:30 a.m., Eastern Time, for the purpose of considering and voting on the following proposals:

PRA will transact no other business at the PRA stockholder meeting except such business as may properly be brought before the PRA stockholder meeting or any adjournment or postponement thereof. The accompanying joint proxy statement/prospectus, including the merger agreement attached thereto as Annex A, contains further information with respect to these matters.

Only holders of record of PRA common stock at the close of business on April 26, 2021 are entitled to notice of and to vote at the PRA stockholder meeting and any adjournments or postponements thereof.

The PRA board of directors has unanimously approved and declared advisable the merger agreement and the transactions contemplated by the merger agreement on the terms and subject to the conditions set forth in the merger agreement. The PRA board of directors unanimously recommends that PRA stockholders vote “FOR” the PRA merger agreement proposal, “FOR” the PRA compensation proposal and “FOR” the PRA adjournment proposal.

Your vote is very important, regardless of the number of shares of PRA common stock you own. We cannot complete the transactions contemplated by the merger agreement without approval of the PRA merger agreement proposal. Assuming a quorum is present, the approval of the PRA merger agreement proposal requires the affirmative vote of a majority of the outstanding shares of PRA common stock entitled to vote on the PRA merger agreement proposal.

Whether or not you plan to attend the PRA stockholder meeting virtually, we urge you to please promptly mark, sign and date the accompanying proxy and return it in the enclosed postage-paid envelope or authorize the individuals

named on the proxy card to vote your shares by calling the toll-free telephone number or by using the Internet as described in the instructions included with the proxy card. If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.

If you have any questions about the merger, please contact PRA Health Sciences, Inc., Attention: Corporate Secretary, 4130 ParkLake Avenue, Suite 400, Raleigh, North Carolina 27612.

If you have any questions about how to vote or direct a vote in respect of your shares of PRA common stock, you may contact our proxy solicitor, MacKenzie Partners, Inc., toll-free at (800) 322-2885 or collect at (212) 929-5500.

Your vote is important. PRA stockholders are requested to complete, date, sign and return the enclosed proxy in the envelope provided, which requires no postage if mailed in the United States, or to submit their votes electronically through the Internet or by telephone.

This joint proxy statement/prospectus incorporates important business and financial information about ICON and PRA from other documents that ICON and PRA have filed with the U.S. Securities and Exchange Commission, which is referred to as the SEC, and that are contained in or incorporated by reference into this joint proxy statement/prospectus. For a listing of documents incorporated by reference into this joint proxy statement/prospectus, please see the section entitled “Where You Can Find More Information” beginning on page 218. This information is available for you free of charge to review through the SEC’s website at www.sec.gov.

Any person may request a copy of this joint proxy statement/prospectus and any of the documents incorporated by reference into this joint proxy statement/prospectus or other information concerning ICON or PRA, without charge, by written or telephonic request directed to the appropriate company or its proxy solicitor at the following contacts:

In order for you to receive timely delivery of the documents in advance of the ICON extraordinary general meeting to be held on June 15, 2021, which is referred to as the ICON EGM, or the special meeting of PRA stockholders to be held on June 15, 2021, which is referred to as the PRA stockholder meeting, as applicable, you must request the information no later than June 1, 2021.

The contents of the websites of the SEC, ICON, PRA or any other entity are not being incorporated into this joint proxy statement/prospectus. The information about how you can obtain certain documents that are incorporated by reference into this joint proxy statement/prospectus at these websites is being provided only for your convenience.

This document, which forms part of a registration statement on Form F-4 filed with the SEC by ICON (File No. 333-254891), constitutes a notice of meeting with respect to the ICON EGM and a prospectus of ICON under Section 5 of the Securities Act of 1933, as amended, which is referred to as the Securities Act, with respect to the ICON ordinary shares to be issued to PRA stockholders pursuant to the Agreement and Plan of Merger, dated as of February 24, 2021, by and among ICON plc, ICON US Holdings Inc., Indigo Merger Sub, Inc. and PRA Health Sciences, Inc., as it may be amended from time to time, which is referred to as the merger agreement.

This document also constitutes a notice of meeting with respect to the PRA stockholder meeting and a proxy statement of PRA under Section 14(a) of the Securities Exchange Act of 1934, as amended, which is referred to as the Exchange Act.

This joint proxy statement/prospectus is not intended to be and is not a prospectus for the purposes of the Companies Act of 2014 of Ireland, the European Union (Prospectus) Regulations 2019 of Ireland (as amended) or the Prospectus Rules issued by the Central Bank of Ireland, and the Central Bank of Ireland has not approved this joint proxy statement/prospectus.

ICON has supplied all information contained or incorporated by reference into this joint proxy statement/prospectus relating to ICON, and PRA has supplied all information contained or incorporated by reference into this joint proxy statement/prospectus relating to PRA. ICON and PRA have both contributed to the information related to the merger contained in this joint proxy statement/prospectus.

You should rely only on the information contained in or incorporated by reference into this joint proxy statement/prospectus. ICON and PRA have not authorized anyone to provide you with information that is different from that contained in or incorporated by reference into this joint proxy statement/prospectus. This joint proxy statement/prospectus is dated April 27, 2021, and you should not assume that the information contained in this joint proxy statement/prospectus is accurate as of any date other than such date unless otherwise specifically provided herein.

Further, you should not assume that the information incorporated by reference into this joint proxy statement/prospectus is accurate as of any date other than the date of the incorporated document. Neither the mailing of this joint proxy statement/prospectus to ICON shareholders or PRA stockholders nor the issuance by ICON of ICON ordinary shares pursuant to the merger agreement will create any implication to the contrary.

This joint proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction.

When used in this joint proxy statement/prospectus, all references to “ICON” refer to ICON plc., a public limited company incorporated under the laws of Ireland, or, where the context requires, to ICON plc, one or more of its consolidated subsidiaries, or to all of them taken as a whole; all references to “US HoldCo” refer to ICON US Holdings Inc., a Delaware corporation and wholly owned subsidiary of ICON; all references to “Merger Sub” refer to Indigo Merger Sub, Inc., a Delaware corporation and wholly owned subsidiary of ICON and US HoldCo, formed for the purpose of effecting the merger as described in this joint proxy statement/prospectus; all references to “PRA” refer to PRA Health Sciences, Inc., a Delaware corporation; all references to “combined company” refer to ICON immediately following completion of the merger and the other transactions contemplated by the merger agreement; all references to “ICON ordinary shares” refer to the ordinary shares of ICON plc, par value €0.06 each; and all references to “PRA common stock” refer to the common stock of PRA, par value $0.01 per share.

This joint proxy statement/prospectus contains or incorporates by reference:

Unless indicated otherwise, financial data presented in this joint proxy statement/prospectus has been taken from the ICON consolidated financial statements and the PRA consolidated financial statements incorporated by reference into this joint proxy statement/prospectus.

This joint proxy statement/prospectus also contains the unaudited pro forma condensed combined financial information of ICON as of and for the year ended December 31, 2020 after giving effect to the merger, referred to in this joint proxy statement/prospectus as the pro forma financial information. See the section of this joint proxy statement/prospectus entitled “Unaudited Pro Forma Condensed Combined Financial Information.”

The pro forma financial information set forth in this joint proxy statement/prospectus has been rounded for ease of presentation. Accordingly, in certain cases, the sum of the numbers in a column in a table may not conform to the total figure given for that column.

For additional information on the presentation of financial information in this joint proxy statement/prospectus, see the ICON consolidated financial statements and the PRA consolidated financial statements, in each case, that are incorporated by reference into this joint proxy statement/prospectus.

The following are answers to certain questions you may have regarding the merger, the merger agreement, the PRA stockholder meeting and the ICON extraordinary general meeting, which is referred to as the ICON EGM. You are urged to carefully read this joint proxy statement/prospectus and the other documents referred to in this joint proxy statement/prospectus in their entirety because this section may not provide all the information that is important to you regarding these matters. Additional important information is contained in the annexes to, and the documents incorporated by reference into, this joint proxy statement/prospectus. You may obtain the information incorporated by reference in this joint proxy statement/prospectus, without charge, by following the instructions under the section entitled “Where You Can Find More Information” beginning on page 218.

If you are in any doubt about this transaction you should consult an independent financial advisor who, if you are obtaining advice in Ireland, is authorized or exempted by the Investment Intermediaries Act 1995, or the European Union (Markets in Financial Instruments) Regulations 2017 (as amended).

At the ICON EGM, ICON shareholders are being asked:

Upon completion of the merger, each share of PRA common stock outstanding immediately prior to the merger, other than excluded shares (as defined in the section entitled “The Merger—Merger Consideration” beginning on page 66) and dissenting shares (as defined in the section entitled “The Merger Agreement—Dissenting Shares” beginning on page 119), will be converted automatically into the right to receive (i) 0.4125 of one ICON ordinary share and (ii) $80.00 in cash, without interest. This exchange ratio is fixed in the merger agreement and will not be adjusted for changes in the market price of either ICON ordinary shares or PRA common stock. The market price of ICON ordinary shares and, as a result, the value of the share consideration that PRA stockholders will receive pursuant to the merger agreement, has been fluctuating since the date that the merger agreement was executed and may continue to fluctuate through the date of the completion of the merger. Accordingly, the market price of ICON ordinary shares when PRA stockholders receive those shares after the merger is completed could be greater than, less than or the same as the market price of ICON ordinary shares on the date the merger agreement was executed, on the date of this joint proxy statement/prospectus or on the date of the stockholder meetings. Stock price changes may result from a variety of factors, including, among others, changes in ICON’s or PRA’s respective businesses, operations or prospects; legislative, regulatory and legal developments in the healthcare industry; general market, industry and economic conditions; and market assessments of the likelihood that the merger will be completed. Because the value of the share consideration will depend on the market price of ICON ordinary shares at the time the merger is completed, PRA stockholders will not know or be able to determine at the time of the PRA stockholder meeting the market value of the merger consideration they would receive upon completion of the merger. Similarly, ICON shareholders will not know or be able to determine at the time of the ICON EGM the market value of the ICON ordinary shares to be issued pursuant to the merger agreement compared to the market value of the shares of PRA common stock that are being exchanged. You are urged to obtain current market quotations for ICON ordinary shares and PRA common stock in determining whether to vote for approval of the share issuance in the case of ICON shareholders or for the adoption of the merger agreement in the case of PRA stockholders. In addition, see the section entitled “Comparative Per Share Market Price Information” beginning on page 31.

The merger is subject to a number of conditions to closing as specified in the merger agreement. These closing conditions include, among others, receipt of the required PRA vote and the required ICON vote; the expiration or termination of the waiting periods applicable to the merger under the HSR Act; obtainment of all required approvals under antitrust and foreign direct investment laws of certain jurisdictions; obtainment of certain consents, approvals and other authorizations of governmental entities free of any burdensome condition; the ICON ordinary shares to be issued to PRA stockholders having been approved for listing on Nasdaq; the effectiveness of the registration statement of which this joint proxy statement/prospectus forms a part and the absence of a stop order or proceedings seeking a stop order; absence of any governmental entity having jurisdiction over any party having enacted, issued, promulgated, enforced, or entered any laws or orders that make illegal, enjoin, or otherwise prohibit consummation of the merger, the ICON share issuance, or the other transactions contemplated by the merger agreement; the accuracy of the representations and warranties of the parties to the merger agreement to the extent required under the merger agreement; the performance in all material respects by each party of all obligations and covenants required to be performed by or complied with prior to the closing date; the receipt by each party of an officer’s certificate of the other party certifying as to certain matters; and that ICON has in place a composition agreement with the Revenue Commissioners of Ireland and a Special Eligibility Agreement for Securities with The Depository Trust Company in respect of the ICON ordinary shares issuable as merger consideration. There can be no assurance as to when these

conditions will be satisfied or waived, if at all, or that other events will not intervene to delay or result in the failure to complete the transaction. For a more complete summary of the conditions that must be satisfied or waived prior to completion of the merger, see the section entitled “The Merger Agreement—Conditions to the Completion of the Merger” beginning on page 140.

If the merger is not completed on a timely basis, or at all, for any reason, including as a result of ICON shareholders failing to approve the share issuance or PRA stockholders failing to adopt the merger agreement, ICON’s and PRA’s respective ongoing businesses may be adversely affected and, without realizing any of the benefits of having completed the merger, ICON and PRA would be subject to a number of risks, including the following:

After the date of the merger agreement and prior to the effective time, the merger agreement restricts ICON and PRA from taking specified actions without the consent of the other party (which consent may not be unreasonably withheld or delayed) and requires that the business of each company and its respective subsidiaries be conducted in all material respects in the ordinary course of business consistent with past practice. These restrictions may prevent ICON or PRA from making appropriate changes to their respective businesses or organizational structures or from pursuing attractive business opportunities that may arise prior to the completion of the merger and could have the effect of delaying or preventing other strategic transactions. Adverse effects arising from the pendency of the merger could be exacerbated by any delays in consummation of the merger or termination of the merger agreement. See the section entitled “The Merger Agreement—Conduct of Business Prior to the Effective Time” beginning on page 123.

ICON intends to fund the cash consideration due pursuant to the merger agreement and to refinance certain of ICON’s and PRA’s existing indebtedness with a combination of cash on hand at ICON and PRA and debt financing. To this end, ICON has entered into a commitment letter pursuant to which certain debt financing sources have committed, subject to customary closing conditions, to provide to ICON a senior secured revolving credit facility in an aggregate principal amount of up to $300.0 million and a senior secured bridge loan facility in an aggregate principal amount of up to $6,060.0 million. However, as of the date of this joint proxy statement/prospectus, neither

ICON nor any of its subsidiaries has entered into definitive agreements for any such debt facilities. There can be no assurance that ICON will be able to secure the debt financings contemplated by the commitment letter. In the event that the debt financing contemplated by the commitment letter is not available, other financing to enable ICON to pay the cash portion of the merger consideration may not be available on acceptable terms, in a timely manner, or at all.

No assurance can be given that the required consents, orders and approvals will be obtained or that the required conditions to the completion of the transaction will be satisfied. Even if all such consents, orders and approvals are obtained and such conditions are satisfied, no assurance can be given as to the terms, conditions and timing of such consents, orders and approvals. For example, these consents, orders and approvals may impose conditions on or require divestitures relating to the divisions, operations or assets of ICON and PRA or may impose requirements, limitations or costs or place restrictions on the conduct of ICON’s or PRA’s business, and if such consents, orders and approvals require an extended period of time to be obtained, such extended period of time could increase the chance that an adverse event occurs with respect to ICON or PRA. Such extended period of time also may increase the chance that other adverse effects with respect to ICON or PRA could occur, such as the loss of key personnel. Even if all necessary approvals are obtained, no assurance can be given as to the terms, conditions and timing of such approvals. For more information, see the sections entitled “The Merger Agreement—Reasonable Best Efforts and Regulatory Approvals” and “The Merger Agreement—Conditions to Completion of the Merger.”

The merger will happen only if certain conditions are met including, among other conditions, the approval of the merger agreement proposal by the required PRA vote, the approval of the share issuance proposal by the required ICON vote and either early termination or expiration of any applicable waiting period or periods under the HSR Act. Many of the conditions are outside the control of ICON and PRA, and both parties also have certain rights to terminate the merger agreement. Accordingly, there may be uncertainty regarding the completion of the merger. This uncertainty may cause customers, suppliers, vendors, strategic partners or others that deal with ICON and/or PRA to delay or defer entering into contracts with ICON and/or PRA or making other decisions concerning ICON and/or PRA or seek to change or cancel existing business relationships with ICON and/or PRA, which could negatively affect ICON’s and PRA’s respective businesses. Any delay or deferral of those decisions or changes in existing agreements could have a material adverse effect on the respective businesses of ICON and PRA, regardless of whether the merger is ultimately completed.

The success of the merger will depend in part on the retention of personnel critical to the business and operations of the combined company due to, for example, their technical skills or management expertise. Competition for qualified personnel can be intense.

Current and prospective employees of ICON and PRA may experience uncertainty about their future role with ICON and PRA until strategies with regard to these employees are announced or executed, which may impair ICON’s and PRA’s ability to attract, retain and motivate key management, clinical, technical, business development, operational and customer-facing employees and other personnel prior to and following the merger. If ICON and PRA are unable to retain personnel, ICON and PRA could face disruptions in their operations, loss of existing customers, loss of key information, expertise or know-how, and unanticipated additional recruitment and training costs. In addition, the loss of key personnel could diminish the anticipated benefits of the merger.

At the effective time, outstanding shares of PRA restricted stock will vest at closing and be converted into the right to receive the merger consideration, and PRA’s outstanding compensatory restricted stock unit and stock option awards will be assumed and converted into corresponding ICON equity awards, generally subject to the same terms and conditions as were applicable to the applicable PRA equity award (including applicable vesting conditions). As discussed further below in “The Merger Agreement—Employee Benefit Plans and Compensation

Matters—Retention Bonus Program” beginning on page 135, pursuant to the terms of the merger agreement, the parties will implement a retention bonus program that will provide cash retention bonus awards to certain employees in connection with the merger. In addition, each of PRA’s executive officers are entitled to receive severance benefits upon a qualifying termination of employment following the completion of the merger. Each of PRA’s executive officers could potentially terminate his or her employment following specified circumstances set forth in the section entitled “Interests of PRA’s Directors and Executive Officers in the Merger—PRA Executive Employment Agreements” beginning on page 167. See the section entitled “Interests of PRA’s Directors and Executive Officers in the Merger” beginning on page 167 for a further discussion of some of these issues.

If key employees of ICON or PRA depart, the integration of the companies may be more difficult and the combined company’s business following the merger may be harmed. Furthermore, the combined company may have to incur significant costs in identifying, hiring and retaining replacements for departing employees and may lose significant expertise and talent relating to the business of each of ICON or PRA, and the combined company’s ability to realize the anticipated benefits of the merger may be adversely affected. In addition, there could be disruptions to or distractions for the workforce and management associated with activities of labor unions or integrating employees into the combined company. Accordingly, no assurance can be given that the combined company will be able to attract or retain key employees of ICON and PRA to the same extent that those companies have been able to attract or retain their own employees in the past.

Either ICON or PRA may terminate the merger agreement under certain circumstances, including, among other reasons, if the merger is not completed by February 24, 2022 (subject, under certain circumstances, to extension to August 24, 2022, or later). In addition, if the merger agreement is terminated under certain circumstances specified in the merger agreement, PRA may be required to pay ICON and US HoldCo a termination fee of $277,000,000 or an expense reimbursement of $100,000,000, including certain circumstances in which the PRA board of directors effects a change of recommendation (as defined in the section entitled “The Merger Agreement—No Solicitation of Takeover Proposals; No Change of Recommendation” beginning on page 127) or PRA enters into an agreement with respect to an takeover proposal (as defined in the section entitled “The Merger Agreement—No Solicitation of Takeover Proposals; No Change of Recommendation” beginning on page 127) following the termination of the merger agreement. In addition, if the merger agreement is terminated under certain circumstances specified in the merger agreement, ICON and US HoldCo may be required to pay PRA a termination fee of $388,000,000 or an expense reimbursement of $120,000,000, including certain circumstances in which the ICON board of directors effects a change of recommendation or ICON enters into an agreement with respect to a takeover proposal following the termination of the merger agreement. See the section entitled “The Merger Agreement—Termination of the Merger Agreement” beginning on page 141 and the section entitled “The Merger Agreement—Termination Fees” beginning on page 142 for a more complete discussion of the circumstances under which the merger agreement could be terminated and when a termination fee may be payable by ICON or PRA.

When considering the recommendations of the boards of directors of ICON or PRA, as applicable, with respect to the proposals described in this joint proxy statement/prospectus, ICON shareholders and PRA stockholders should be aware that the directors and executive officers of each of ICON and PRA may have interests in the merger and have arrangements that are different from, or in addition to, those of ICON shareholders and PRA stockholders generally. These interests and arrangements may create potential conflicts of interest.

With respect to directors and executive officers of ICON, these interests and arrangements include the continued employment of such executive officers by ICON after the merger and the continued service of such directors on the board of directors of ICON after the merger. The ICON board of directors was aware of these interests and considered these interests, among other matters, when it approved and declared advisable the merger agreement and the transactions contemplated thereby on the terms and subject to the conditions set forth in the merger agreement and recommended that ICON shareholders approve the share issuance. The interests of ICON directors and executive officers are described in more detail in the section entitled “Interests of ICON’s Directors and Executive Officers in the Merger” beginning on page 166.

With respect to directors and executive directors of PRA, these interests and arrangements include the continued employment of certain executive officers by the combined company, the appointment of certain PRA directors as

directors of the combined company, the treatment in the merger of outstanding equity, equity-based and incentive awards, severance arrangements, other compensation and benefit arrangements and the right to continued indemnification of former PRA directors and officers by the combined company. The PRA board of directors was aware of these interests and considered these interests, among other matters, when it approved and declared advisable the merger agreement, the merger and the transactions contemplated thereby on the terms and subject to the conditions set forth in the merger agreement, determined that the merger agreement, the merger and the transactions contemplated by the merger agreement were fair to, and in the best interests of, PRA and PRA stockholders and recommended that PRA stockholders adopt the merger agreement. The interests of PRA directors and executive officers are described in more detail in the section entitled “Interests of PRA’s Directors and Executive Officers in the Merger” beginning on page 167.

ICON or PRA may determine to waive, in whole or part, one or more of the conditions of its obligations to consummate the merger. ICON and PRA currently expect to evaluate the materiality of any waiver and its effect on ICON or PRA stockholders, as applicable, in light of the facts and circumstances at the time to determine whether any amendment of this joint proxy statement/prospectus or any re-solicitation of proxies or voting cards is required in light of such waiver. Any determination whether to waive any condition to the merger or as to re-soliciting stockholder approval or amending this joint proxy statement/prospectus as a result of a waiver will be made by ICON or PRA, as applicable, at the time of such waiver based on the facts and circumstances as they exist at that time.

The merger agreement contains “no shop” provisions that restrict PRA’s ability to, among other things (each as described under the section entitled “The Merger Agreement—No Solicitation of Takeover Proposals; No Change of Recommendation” beginning on page 127):

Furthermore, there are only limited exceptions to the requirement under the merger agreement that PRA’s board of directors withhold or withdraw (or modify, amend or qualify in a manner adverse to ICON or Merger Sub), or propose publicly to withhold or withdraw (or modify, amend or qualify in a manner adverse to ICON or Merger Sub), the PRA recommendation (as defined in the section entitled “The Merger Agreement—Representations and Warranties” beginning on page 120.) Although PRA’s board of directors is permitted to effect a change of recommendation, after complying with certain procedures set forth in the merger agreement, in response to a takeover proposal if it determines in good faith that a failure to do so would be inconsistent with its fiduciary duties, its doing so would entitle ICON and US HoldCo to terminate the merger agreement and collect a termination fee from PRA in the amount of $277,000,000. For more information, see the sections titled “The Merger Agreement—Termination of the Merger Agreement” beginning on page 141 and “The Merger Agreement—Termination Fees” beginning on page 142.

These provisions could discourage a potential competing acquirer from considering or proposing an acquisition or merger, even if it were prepared to pay consideration with a higher value than that implied by the merger consideration, or might result in a potential competing acquirer proposing to pay a lower per share price than it might otherwise have proposed to pay because of the added expense of the termination fee.

ICON and PRA have incurred and expect to incur a number of non-recurring costs associated with the merger. These costs and expenses include fees paid to financial, legal and accounting advisors, systems consolidation costs, severance and other potential employment-related costs, including payments that may be made to certain ICON executives and certain PRA executives, filing fees, printing expenses and other related charges. Some of these costs are payable by ICON and PRA regardless of whether the merger is completed. There are also a large number of processes, policies, procedures, operations, technologies and systems that must be integrated in connection with the merger and the integration of the two companies’ businesses. While both ICON and PRA have assumed that a certain level of expenses would be incurred in connection with the merger and the other transactions contemplated by the merger agreement, there are many factors beyond their control that could affect the total amount or the timing of the integration and implementation expenses.

There may also be additional unanticipated significant costs in connection with the merger that the combined company may not recoup. These costs and expenses could reduce the realization of efficiencies, strategic benefits and additional income ICON and PRA expect to achieve from the merger. Although ICON and PRA expect that these benefits will offset the transaction expenses and implementation costs over time, this net benefit may not be achieved in the near term or at all.

Neither ICON nor PRA has obtained, nor will obtain, an updated opinion regarding the fairness, from a financial point of view, of the merger consideration as of the date of this joint proxy statement/prospectus or prior to the completion of the merger. The financial advisors’ opinions do not speak as of any date other than the respective dates of such opinions. For a more complete description of the above-described opinions, please refer to “The Merger—Opinion of ICON’s Financial Advisor” beginning on page 78 and “The Merger—Opinions of PRA’s Financial Advisors” beginning on page 88.

The unaudited financial forecasts included in this joint proxy statement/prospectus were provided by ICON and/or PRA to their respective boards of directors and financial advisors in connection with each party’s consideration of the merger agreement. Such unaudited financial forecasts were prepared based on assumptions regarding future operating cash flows, expenditures, income and other variables that may not be realized in full, at all, or within projected timeframes and the combined company’s actual results may differ materially and adversely from the unaudited financial forecasts. The unaudited financial forecasts were not prepared with a view toward public disclosure. Neither ICON nor PRA undertakes any obligation to update any unaudited prospective financial

information included in this joint proxy statement/prospectus except as may be required by applicable law. For more information, see the sections entitled “ICON Unaudited Financial Forecasts” beginning on page 107 and “PRA Unaudited Financial Forecasts” beginning on page 109.

The unaudited pro forma condensed combined financial information included in this joint proxy statement/prospectus has been prepared using the consolidated historical financial statements of ICON and PRA, is presented for illustrative purposes only, and should not be considered to be an indication of the results of operations or financial condition of the combined company following the merger and the other transactions contemplated by the merger agreement (which, together with the merger, are referred to collectively as the transactions). In addition, the pro forma financial information included in this joint proxy statement/prospectus is based in part on certain assumptions regarding the transactions. These assumptions may not prove to be accurate, and other factors may affect the combined company’s results of operations or financial condition following the completion of the transactions. Accordingly, the historical and pro forma financial information included in this joint proxy statement/prospectus does not necessarily represent the combined company’s results of operations and financial condition had ICON and PRA operated as a combined entity during the periods presented, or of the combined company’s results of operations and financial condition following completion of the transactions. The combined company’s potential for future business success and operating profitability must be considered in light of the risks, uncertainties, expenses and difficulties typically encountered by recently combined companies. For more information, see the section entitled “Unaudited Pro Forma Condensed Combined Financial Information” beginning on page 146.

PRA has contracts with customers, suppliers, vendors, landlords, licensors and other business partners which may require PRA to obtain consent from these other parties in connection with the merger. If these consents cannot be obtained, the combined company may suffer a loss of potential future revenue and may lose rights that are material to its business and the business of the combined company. In addition, third parties with whom ICON or PRA currently have relationships may terminate or otherwise reduce the scope of their relationship with either party in anticipation of the merger. Any such disruptions could limit the combined company’s ability to achieve the anticipated benefits of the merger. The adverse effect of such disruptions could also be exacerbated by a delay in the completion of the merger or the termination of the merger agreement.

Shareholder lawsuits are often brought against public companies that have entered into merger agreements. Even if the lawsuits are without merit, defending against these claims can result in substantial costs and divert management time and resources. Lawsuits filed in connection with the merger against ICON, PRA, US HoldCo, Merger Sub and/or their respective directors and officers could prevent or delay the consummation of the merger, including through an injunction, and result in additional costs to ICON and PRA. The ultimate resolution of any lawsuits cannot be predicted, and an adverse ruling in any such lawsuit may cause the merger to be delayed or not to be completed, which could cause ICON and PRA not to realize some or all of the anticipated benefits of the merger. The defense or settlement of any lawsuit or claim that remains unresolved at the time the merger is consummated may adversely affect the combined company’s business, financial condition, results of operations and cash flows. ICON and PRA cannot currently predict the outcome of or reasonably estimate the possible loss or range of loss from any lawsuits or claims. As of the date of this joint proxy statement/prospectus, ICON and PRA are only aware of four (4) shareholder lawsuits filed in connection with the merger. For information with respect to the existing lawsuits relating to the merger, see the section entitled “The Merger—Litigation Relating to the Merger” beginning on page 114. Additional lawsuits may be filed in connection with the merger in the future.

Upon consummation of the merger, the rights of PRA stockholders, who will become shareholders of ICON, will be governed by the ICON constitution. The rights associated with PRA common stock are different from the rights associated with ICON ordinary shares. See the section entitled “Comparison of Stockholders’ Rights” beginning on page 184 for a discussion of these differences.

As discussed in more detail in “Certain United States Federal Income Tax Considerations—Certain U.S. Federal Income Tax Considerations Related to the Merger—Tax Characterization of the Merger—The US HoldCo Acquisition,” if the PRA stockholders immediately before the merger are deemed, immediately after the merger, to own collectively 50% or more (by vote or value) of the outstanding stock of ICON (taking into account the ICON ordinary shares being issued to the PRA stockholders as merger consideration), Section 304 of the Code (“Section 304”) may require the cash portion of the merger consideration deemed to have been received from US HoldCo to be treated, for U.S. federal income tax purposes, as a dividend rather than as proceeds of a sale of PRA common stock. Even if Section 304 were to apply, such dividend treatment generally should apply to any particular PRA stockholder only if the percentage of PRA’s outstanding common stock constructively owned by such stockholder immediately after the merger (through such stockholder’s actual and constructive ownership of ICON ordinary shares immediately after the merger) does not represent a meaningful reduction relative to such stockholder’s actual and constructive equity ownership percentage in PRA’s outstanding common stock immediately prior to the merger. However, because it is generally impracticable to ascertain the identity of all of the stockholders of a publicly-traded corporation (or the extent to which any identified stockholder constructively owns additional shares), it will be impracticable to determine whether Section 304 will actually apply to the merger or whether, if Section 304 were to apply, any particular stockholder would qualify for sale (as opposed to dividend) treatment. Moreover, as discussed in more detail in “Certain United States Federal Income Tax Considerations—Certain U.S. Federal Income Tax Considerations Related to the Merger—Tax Characterization of the Merger—The Deemed PRA Redemption,” although as a technical matter a portion of the cash consideration should be deemed to have been received from PRA (as opposed to US HoldCo) and, therefore not subject to Section 304 treatment, ICON is not yet able to determine which portion of the cash consideration should be deemed to have been received from PRA or US HoldCo, respectively, and may not be able to do so prior to closing.

Given the foregoing uncertainty, regardless of whether sale (as opposed to dividend) treatment is appropriate in a PRA stockholder’s particular case, ICON intends to assume (both for purposes of withholding (with respect to non-U.S. PRA stockholders) and information reporting to the IRS (with respect to all PRA stockholders)), and other applicable withholding agents are likely to assume (for such purposes), that (1) Section 304 will apply to the merger, (2) dividend treatment (as opposed to sale treatment) will apply to each PRA stockholder and (3) such dividend treatment will apply to all of the cash consideration (other than cash in lieu of fractional ICON ordinary shares).

If dividend treatment were to apply, (a) a U.S. stockholder of PRA generally would be subject to dividend treatment with respect to the applicable cash consideration without reduction for any portion of the stockholder’s adjusted tax basis in its PRA shares and (b) a non-U.S. stockholder of PRA generally would be subject to a U.S. federal withholding tax of 30% (or a lower applicable treaty rate) with respect to the applicable cash consideration without reduction for any portion of the stockholder’s adjusted tax basis in its PRA shares.

PRA stockholders should consult their tax advisors about Section 304, including the possibility of avoiding any adverse tax consequences under Section 304 by selling their shares of PRA common stock in advance of the merger and, if a stockholder of PRA decides not to sell its PRA shares in advance of the merger, the possibility of reporting all of the cash consideration as sale proceeds on the PRA stockholder’s U.S. federal income tax return, and/or seeking a refund of any excess withholding tax from the IRS, if sale (as opposed to dividend) treatment is appropriate in a particular case.

While ICON believes that the intended tax treatment discussed in “Certain United States Federal Income Tax Considerations—Certain U.S. Federal Income Tax Considerations Related to the Merger—Tax Characterization of the Merger” (i.e., a partial purchase of the PRA common stock by ICON (for ICON ordinary shares) and a partial purchase of PRA common stock by US HoldCo (for cash)) should be respected, this position is not entirely free from doubt and the IRS could assert that all of the outstanding common stock of PRA (other than the portion deemed to have been redeemed by PRA) was acquired solely by US HoldCo or, alternatively, solely by ICON.

If the IRS were to successfully recharacterize the merger as an acquisition solely by US HoldCo, all of the merger consideration (i.e., the ICON ordinary shares and the cash, as opposed to only the cash) would be subject to possible deemed dividend treatment under Section 304 (as described below under “Certain United States Federal Income Tax Considerations—Certain U.S. Federal Income Tax Considerations Related to the Merger—Tax Characterization of the Merger—The US HoldCo Acquisition”).

If the IRS were to successfully recharacterize the merger as an acquisition solely by ICON, a “U.S. Holder” (as defined under “Certain United States Federal Income Tax Considerations”) would be treated as having transferred a portion of each share of its PRA common stock for cash to ICON (a non-U.S. corporation) as opposed to US HoldCo (a U.S. corporation), which recharacterized transfer could, under Section 367(a) of the Code, cause U.S. Holders to recognize gain on such transfer as a taxable sale of such stock in addition to being subject to dividend treatment under Section 304 with respect to the same transfer, resulting in a greater total tax on the merger than if the intended tax treatment were respected.

PRA stockholders should consult their tax advisors about the foregoing issues, including the possibility of avoiding any adverse tax consequences described above by selling their PRA common stock in advance of the merger.

Following completion of the merger and the other transactions contemplated by the merger agreement, the combined company will have a substantial amount of debt. ICON expects to borrow approximately $6,441.9 million in order to pay PRA stockholders the cash consideration due to them as merger consideration under the merger agreement, pay related fees and transaction costs in connection with the transactions, and refinance certain existing indebtedness of PRA. ICON’s consolidated borrowings were $348.5 million as at December 31, 2020. The combined company’s pro forma borrowings as at December 31, 2020, if the transactions had been completed on December 31, 2020, would have been approximately $6,287.7 million. This increased level of borrowings could adversely affect the combined company in a number of ways, including, but not limited to, by making it more difficult for the combined company to satisfy its obligations with respect to its debt or to its trade or other creditors, requiring a substantial portion of the combined company’s cash flows from operations for the payment of interest on the combined company’s debt, reducing the combined company’s flexibility to respond to changing business and economic conditions, and reducing funds available for the combined company’s investments in research and development, capital expenditures and other activities. If ICON cannot service its debt, it may have to take actions such as selling assets, seeking additional debt or equity, or reducing or delaying capital expenditures, strategic acquisitions, investments and alliances.

In addition, ICON’s increased level of indebtedness could adversely affect ICON’s credit rating which could result in increased borrowing costs for the combined company in the future. No assurances can be made that ICON will be able to refinance any indebtedness incurred in connection with the merger on terms acceptable to it or at all.

As a result of the merger, the combined company may experience strain in relationships with customers and suppliers that may harm the combined company’s business and results of operations. Certain customers or suppliers may seek to terminate or modify contractual obligations following the merger (whether or not contractual rights are triggered as a result of the merger) or may decline to award new business to, or renew existing contracts or enter into

new long-term agreements with, the combined company. There can be no guarantee that customers and suppliers will remain with or continue to have a relationship with the combined company or do so on the same or similar contractual terms following the merger or that the combined company will be able to attract new customers. If any of the customers or suppliers seek to terminate or modify contractual obligations or discontinue the relationship with the combined company, or if the combined company is unable to attract new customers, then the combined company’s business and results of operations may be harmed.

The success of the merger will depend on, among other things, the ability of ICON and PRA to combine their businesses in a manner that facilitates growth opportunities and realizes cost savings. ICON and PRA have entered into the merger agreement because each believes that the merger and the other transactions contemplated by the merger agreement are fair to and in the best interests of its respective stockholders and that combining the businesses of ICON and PRA will produce benefits and cost savings.

However, ICON and PRA must successfully combine their respective businesses in a manner that permits these benefits to be realized. In addition, the combined company must achieve the anticipated growth and cost savings without adversely affecting current revenues and investments in future growth. If the combined company is not able to successfully achieve these objectives, the anticipated benefits of the merger may not be realized fully, or at all, or may take longer to realize than expected.

An inability to realize the full extent of the anticipated benefits of the merger and the other transactions contemplated by the merger agreement, as well as any delays encountered in the integration process, could have an adverse effect upon the revenues, level of expenses and operating results of the combined company, which may adversely affect the value of the ordinary shares of the combined company after the completion of the merger.

In addition, the actual integration may result in additional and unforeseen expenses, and the anticipated benefits of the integration plan may not be realized. Actual growth and cost savings, if achieved, may be lower than what ICON and PRA expect and may take longer to achieve than anticipated. If ICON and PRA are not able to adequately address integration challenges, they may be unable to successfully integrate their operations or realize the anticipated benefits of the integration of the two companies.

ICON and PRA have operated and, until the completion of the merger, will continue to operate independently. There can be no assurances that their businesses can be integrated successfully. It is possible that the integration process could result in the loss of key ICON employees or key PRA employees, the loss of customers, the disruption of either company’s or both companies’ ongoing businesses, inconsistencies in standards, controls, procedures and policies, unexpected integration issues, higher than expected integration costs and an overall post-completion integration process that takes longer than originally anticipated. In addition, due to the integration process, customers may be slow to award new business to the combined company or may not award new business to the combined company at all. Specifically, the following issues, among others, must be addressed in integrating the operations of ICON and PRA in order to realize the anticipated benefits of the merger so the combined company performs as expected:

In addition, at times the attention of certain members of either company’s or both companies’ management and resources may be focused on completion of the merger and the integration of the businesses of the two companies and diverted from day-to-day business operations, which may disrupt each company’s ongoing business and the business of the combined company.

ICON expects to generate run rate cost synergies of approximately $150 million and tax savings from the combined effective tax rate decreasing to 14% both to be realized within approximately four years after completion of the merger. ICON’s ability to achieve such estimated cost and tax synergies in the timeframe described, or at all, is subject to various assumptions by ICON’s management, which may or may not prove to be accurate, as well as the incurrence of costs in ICON’s operations that offset all or a portion of such cost synergies. As a consequence, ICON may not be able to realize all of these cost and tax synergies within the timeframe expected or at all. In addition, ICON may incur additional or unexpected costs in order to realize these cost and tax synergies. ICON’s ability to realize tax synergies is subject to uncertainties. See “—There is a risk that, as a result of the merger, either (1) ICON could be treated as having become a U.S. corporation for U.S. federal income tax purposes or (2) ICON and its affiliates could become subject to certain adverse U.S. federal income tax rules.” Failure to achieve the expected cost and tax synergies could significantly reduce the expected benefits associated with the merger. In addition, ICON has incurred and will incur substantial expenses in connection with completion of the merger, including the costs and expenses of preparing and filing the Form F-4 Registration Statement that contains this joint proxy statement/prospectus with the SEC. ICON expects to continue to incur non-recurring costs associated with consummating the merger, combining the operations of the two companies and achieving the desired cost synergies. These fees and costs have been, and will continue to be, substantial. The substantial majority of nonrecurring expenses will consist of transaction costs related to the merger and include, among others, fees paid to financial, legal and accounting advisors, employee benefit costs and filing fees. Such costs, as well as other unanticipated costs and expenses, could have a material adverse effect on the financial condition and operating results of ICON following the completion of the merger and many of these costs will be borne by ICON even if the merger is not completed.

Upon completion of the merger, ICON expects to issue approximately 28,831,766 ICON ordinary shares to PRA stockholders pursuant to the merger agreement. As a result, it is expected that, immediately after completion of the transaction, former PRA stockholders will own approximately 34% of the outstanding ICON ordinary shares. Additional ICON ordinary shares may be issued from time to time following the merger to holders of PRA equity awards on the terms set forth in the merger agreement. See the section of this joint proxy statement/prospectus entitled “The Merger Agreement—Treatment of PRA Equity Awards” beginning on page 117 for a more detailed explanation. Consequently, current ICON shareholders in the aggregate will have less influence over the management and policies of ICON than they currently have over the management and policies of ICON, and PRA stockholders in the aggregate will have significantly less influence over the management and policies of ICON than they currently have over the management and policies of PRA.

The combined company may be exposed to increased litigation from stockholders, customers, suppliers, consumers and other third parties due to the combination of ICON’s business and PRA’s business following the merger. Such litigation may have an adverse impact on the combined company’s business and results of operations or may cause disruptions to the combined company’s operations.

Upon completion of the transaction, PRA stockholders will become ICON shareholders. ICON’s and PRA’s businesses differ and, accordingly, the results of operations of the combined company will be affected by some factors that are different from those currently or historically affecting the results of operations of ICON and those currently or historically affecting the results of operations of PRA. For a discussion of the businesses of ICON and PRA and of some important factors to consider in connection with those businesses, see the documents incorporated by reference in this joint proxy statement/prospectus and referred to in the section entitled “Where You Can Find Additional Information” beginning on page 218.

ICON expects that it will issue up to approximately 28,831,766 ICON ordinary shares in connection with the merger. The issuance of these new ICON ordinary shares and the sale of additional ICON ordinary shares that may become eligible for sale in the public market from time to time could have the effect of depressing the market price for ICON ordinary shares. The increase in the number of ICON ordinary shares may lead to sales of such ICON ordinary shares or the perception that such sales may occur, either of which may adversely affect the market for, and the market price of, ICON ordinary shares.

The market price of ICON ordinary shares may decline as a result of the transaction if, among other things, the combined company is unable to achieve the expected growth in revenues and earnings, or if the cost savings estimates in connection with the integration of ICON’s and PRA’s businesses are not realized or if the transaction costs related to the transaction are greater than expected. The market price also may decline if the combined company does not achieve the perceived benefits of the transaction as rapidly or to the extent anticipated by the market or if the effect of the transaction on the combined company’s financial position, results of operations or cash flows is not consistent with the expectations of financial or industry analysts.

ICON currently does not expect to declare dividends on ICON ordinary shares and has not done so in the past. Any determination to declare or pay dividends in the future will be at the discretion of the ICON board of directors, subject to relevant laws and dependent on a number of factors, including ICON’s earnings, capital requirements and overall financial condition. Therefore, the only opportunity for PRA stockholders to achieve a return on the ICON ordinary shares received as share consideration may be if the market price of ICON ordinary shares appreciates and shares received as share consideration are sold at a price higher than the implied value of the share consideration. The market price for ICON ordinary shares may not appreciate and may fall below the implied value of the share consideration.

Transfers of ICON ordinary shares effected by means of the transfer of book entry interests in DTC should not be subject to Irish stamp duty where ICON ordinary shares are traded through DTC, either directly or through brokers that hold such shares on behalf of customers through DTC. However, if you hold ICON ordinary shares as of record rather than beneficially through DTC, any transfer of ICON ordinary shares could be subject to Irish stamp duty

(currently at the rate of 1% of the higher of the price paid or the market value of the shares acquired). Payment of Irish stamp duty is generally a legal obligation of the transferee. The potential for Irish stamp duty to arise could adversely affect the price of ICON ordinary shares. For more information, see “Irish Tax Consequences—Stamp Duty.”

As a foreign private issuer under the Exchange Act, ICON is exempt from certain rules under the Exchange Act, and is not required to file periodic reports and financial statements with the SEC as frequently or as promptly as companies whose securities are registered under the Exchange Act but are not foreign private issuers, or to comply with Regulation FD, which restricts the selective disclosure of material non-public information. In addition, ICON is exempt from certain disclosure and procedural requirements applicable to proxy solicitations under Section 14 of the Exchange Act. The members of the ICON management board, officers and principal shareholders are exempt from the reporting and “short-swing” profit recovery provisions of Section 16 of the Exchange Act. Accordingly, there may be less publicly available information concerning ICON than there is for companies whose securities are registered under the Exchange Act but are not foreign private issuers, and such information may not be provided as promptly as it is provided by such companies. In addition, certain information may be provided by ICON in accordance with Irish law, which may differ in substance or timing from such disclosure requirements under the Exchange Act. Further, as a foreign private issuer, under Nasdaq rules, ICON is subject to less stringent corporate governance requirements. Subject to certain exceptions, the rules of Nasdaq permit a foreign private issuer to follow its home country practice in lieu of the listing requirements of Nasdaq, including, for example, certain internal controls as well as board, committee and director independence requirements. Accordingly, if ICON remains a foreign private issuer after the merger, you may not have the same protections afforded to shareholders of companies that are required to comply with all of the Nasdaq corporate governance requirements. See the section entitled “Governance of ICON after the Merger—Certain exemptions from Nasdaq corporate governance requirements” beginning on page 112 for a discussion of ICON’s corporate governance practices that differ from those followed by issuers that are not foreign private issuers under Nasdaq listing standards.

ICON is organized under the laws of Ireland. A substantial portion of ICON’s assets are located outside the United States, and many of ICON’s directors and officers and some of the experts named in this joint proxy statement/prospectus are residents of jurisdictions outside of the United States and the assets of such persons may be located outside of the United States. As a result, it may be difficult for investors to effect service within the United States upon ICON and those directors, officers and experts, or to enforce judgments obtained in U.S. courts against ICON or such persons either inside or outside of the United States, or to enforce in U.S. courts judgments obtained against ICON or such persons in courts in jurisdictions outside the United States, in any action predicated upon the civil liability provisions of the federal securities laws of the United States.

There is no certainty that civil liabilities predicated solely upon the federal securities laws of the United States can be enforced in Ireland, whether by original action or by seeking to enforce a judgment of U.S. courts. In addition, punitive damages awards in actions brought in the United States or elsewhere may be unenforceable in Ireland.

For U.S. federal income tax purposes, a corporation generally is considered to be a domestic corporation (a “U.S. corporation”), which is subject to U.S. federal income tax on its worldwide income, only if such corporation is organized or incorporated under the laws of the United States, any state thereof or the District of Columbia. Accordingly, because ICON is an Irish incorporated entity, it generally would be classified as a non-U.S. corporation

for U.S. federal income tax purposes. However, the so-called “anti-inversion rules” of Section 7874 of the Code (“Section 7874”) provide an exception to this general rule under which, as a result of the merger, either (1) ICON might be treated as having become a U.S. corporation for U.S. federal income tax purposes or (2) ICON might remain a non-U.S. corporation for U.S. federal income tax purposes but ICON and its affiliates might become subject to certain adverse U.S. federal income tax rules.

While ICON believes that Section 7874 should not apply to ICON as a result of the merger, due to the factual and legal complexity of Section 7874 and the limited authority interpreting it, there can be no assurance that ICON’s view, if challenged, would be sustained. Moreover, it is possible that a future change in law could expand the scope of Section 7874 on a retroactive basis. In this regard, (1) a bill recently introduced in Congress (entitled the “No Tax Breaks for Outsourcing Act”) proposes a change to the anti-inversion rules that would, if enacted in its current form, retroactively pose a significant risk that Section 7874 would cause ICON to become a U.S. corporation as a result of the merger (which risk would depend on factors outside of ICON’s control, including the trading price of ICON common stock on the closing date) and (2) on April 7, 2021, the U.S. Treasury Department released the “Made in America Tax Plan,” which announced President Biden’s proposal to adopt such change, but was silent on whether such change would apply retroactively. ICON believes that, based on the current facts (including the current trading price of ICON common stock), Section 7874, as amended by the proposed bill in its current form, would not apply, but due to the factual and legal uncertainty with respect to Section 7874, there can be no assurance that ICON’s view, if challenged, would be sustained. The application of Section 7874 to ICON could have a materially adverse impact on the value of (and the U.S. federal income tax consequences of owning) ICON ordinary shares.

PRA stockholders should review “Certain United States Federal Income Tax Considerations—Certain U.S. Federal Income Tax Considerations Related to the Ownership and Disposition of ICON Ordinary Shares Received in the Merger—Possible Application of Section 7874 to ICON,” for a more detailed discussion of Section 7874.

ICON’s business is and will continue to be subject to the risks described in the sections entitled “Risk Factors” in ICON’s Annual Report on Form 20-F for the year ended December 31, 2020 and in other documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 218 for the location of information incorporated by reference into this joint proxy statement/prospectus.

PRA’s business is and will continue to be subject to the risks described in the sections entitled “Risk Factors” in PRA’s Annual Report on Form 10-K for the year ended December 31, 2020, and in other documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 218 for the location of information incorporated by reference into this joint proxy statement/prospectus.

ICON plc is a clinical research organization, founded in Dublin, Ireland in 1990. Over thirty years ICON has grown significantly to become a leading global provider of outsourced development and commercialization services to pharmaceutical, biotechnology, medical device and government and public health organizations. ICON’s mission is to help its clients to accelerate the development of drugs and devices that save lives and improve quality of life. ICON is a public limited company incorporated in Ireland and operates under the Irish Companies Act 2014. The principal executive office and phone number of ICON is:

ICON US Holdings Inc. is a Delaware corporation and a wholly owned subsidiary of ICON. US HoldCo was formed in 2013 and acts primarily as a holding company for ICON’s US subsidiaries.

Indigo Merger Sub, Inc., a Delaware corporation and a wholly owned subsidiary of ICON and US HoldCo, was formed solely for the purpose of facilitating the merger. Merger Sub has not carried on any activities or operations to date, except for those activities incidental to its formation and undertaken in connection with the merger and the other transactions contemplated by the merger agreement. By operation of the merger, Merger Sub will be merged with and into PRA, with PRA surviving the merger as a wholly owned subsidiary of ICON and US HoldCo. The principal executive office and phone number of each of US HoldCo and Merger Sub is:

ICON ordinary shares are listed on Nasdaq under the ticker symbol “ICLR.”

For more information about ICON, please visit ICON’s Internet website at https://iconplc.com. ICON’s Internet website address is provided as an inactive textual reference only. The information contained on ICON’s Internet website or accessible through it (other than the documents incorporated by reference herein) does not constitute a part of this joint proxy statement/prospectus or any other report or document on file with or furnished to the SEC. Additional information about ICON is included in the documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 218.

PRA Health Sciences, Inc. is one of the world’s leading global contract research organizations by revenue, providing outsourced clinical development and data solution services to the biotechnology and pharmaceutical industries. PRA’s global clinical development platform includes more than 75 offices across North America, Europe, Asia, Latin America, Africa, Australia and the Middle East and more than 17,500 employees worldwide. Since 2000, PRA has participated in approximately 4,000 clinical trials worldwide. In addition, PRA has participated in the pivotal or supportive trials that led to U.S. Food and Drug Administration or international regulatory approval of more than 95 drugs. PRA’s principal executive offices are located in Raleigh, North Carolina. The principal executive office and phone number of ICON is:

PRA common stock is listed on Nasdaq under the ticker symbol “PRAH.”

For more information about PRA, please visit PRA’s Internet website at http://www.prahs.com. PRA’s internet website address is provided as an inactive textual reference only. The information contained on PRA’s internet website or accessible through it (other than the documents incorporated by reference herein) do not constitute a part of this joint proxy statement/prospectus or any other report or document on file with or furnished to the SEC. Additional information about PRA is included in the documents incorporated by reference into this joint proxy statement/prospectus. See the section entitled “Where You Can Find More Information” beginning on page 218.

This joint proxy statement/prospectus is being mailed on or about April 29, 2021, to holders of record of ICON ordinary shares as of the close of business April 26, 2021, and constitutes notice of the ICON EGM in conformity with the requirements of the Irish Companies Act and ICON’s constitution.

This joint proxy statement/prospectus is being provided to ICON shareholders as part of a solicitation of proxies by the ICON board of directors for use at the ICON EGM and at any adjournments or postponements of the ICON EGM. ICON shareholders are encouraged to read the entire document carefully, including the annexes to and documents incorporated by reference into this document, for more detailed information regarding the merger agreement and the transactions contemplated by the merger agreement.

The ICON EGM will be held on Tuesday, June 15, 2021, beginning at 3:30 p.m., Dublin time, at ICON’s global headquarters in South County Business Park, Leopardstown, Dublin 18, Ireland.

ICON encourages shareholders to vote by Internet, by mail or by telephone, rather than attend the ICON EGM in person in light of the public health concerns related to COVID-19. Shareholders are referred to the section below entitled “Attending the ICON EGM” for more information.

The purposes of the ICON EGM are as follows, each as further described in this joint proxy statement/prospectus:

The ICON board of directors unanimously recommends that ICON shareholders vote:

After careful consideration, the ICON board of directors, unanimously (i) determined that the merger agreement and the transactions contemplated thereby are fair to, and in the best interests of, ICON and its stockholders, (ii) directed that the share issuance be submitted to ICON shareholders for their approval and (iii) recommended that ICON shareholders vote in favor of the approval of the share issuance.

See also the section entitled “The Merger—ICON’s Reasons for the Merger” beginning on page 78.

The ICON board of directors has fixed the close of business on April 26, 2021 as the record date of the ICON EGM. If you were a holder of record of ICON ordinary shares as of the close of business on the record date you are entitled to receive notice of and to vote at the ICON EGM or any adjournments or postponements thereof. You are entitled to one vote for each ICON ordinary share that you owned as of the close of business on the ICON record date. As of the close of business on the ICON record date, 52,860,690 ICON ordinary shares were outstanding and entitled to vote at the ICON EGM.

A quorum of shareholders is necessary to conduct the ICON EGM. The presence of three (3) or more shareholders (present by proxy or in person) is necessary to constitute a quorum. Abstentions and broker non-votes will be counted as present for purposes of determining whether there is a quorum. If a quorum is not present, the ICON EGM will be adjourned.

Under Nasdaq rules, banks, brokers and other nominees may use their discretion to vote “uninstructed” shares (i.e., shares of record held by banks, brokers or other nominees, but with respect to which the beneficial owner of such shares has not provided instructions on how to vote on a particular proposal) with respect to matters that are considered to be “routine,” but not with respect to “non-routine” matters. A “broker non-vote” occurs on an item when (i) a bank, broker or other nominee has discretionary authority to vote on one or more proposals to be voted on at a meeting of stockholders, but is not permitted to vote on other proposals without instructions from the beneficial owner of the shares and (ii) the beneficial owner fails to provide the bank, broker or other nominee with such instructions. All of the proposals currently scheduled for consideration at the ICON EGM are “non-routine” matters. Because none of the proposals currently scheduled to be voted on at the ICON EGM are routine matters for which brokers may have discretionary authority to vote, ICON does not expect there to be any broker non-votes at the ICON EGM. As a result, if you hold your ICON ordinary shares in “street name,” your shares will not be represented and will not be voted on any matter unless you affirmatively instruct your bank, broker or other nominee how to vote your shares in one of the ways indicated by your bank, broker or other nominee. It is therefore critical that you cast your vote by instructing your bank, broker or other nominee on how to vote.

If you fail to vote, fail to submit a proxy or fail to return a voting instruction card instructing your broker, bank or other nominee how to vote on any of the ICON proposals, this will have no effect on the vote count for such proposal but, in the case of broker non votes only, will count towards determining whether a quorum is present.

If you respond with an “abstain” vote on any of the ICON proposals, this will have no effect on the vote count for any such proposal, but will count towards determining whether a quorum is present. Executed but unvoted proxies will be voted in accordance with the recommendations of the ICON board of directors. If additional votes must be solicited to approve the share issuance, it is expected that the meeting will be adjourned to solicit additional proxies.

Approval of each of the ICON proposals requires the affirmative vote of a majority of the votes cast, either in person or by proxy, by shareholders entitled to vote on the ICON share issuance proposal at the ICON EGM. Because the vote required to approve each of the ICON proposals is based on votes properly cast at the ICON EGM, abstentions, along with failures to vote, will have no effect on such proposals.

As of the record date, ICON directors and executive officers, and their affiliates, as a group, owned and were entitled to vote 638,389 ICON ordinary shares, or approximately 1.21% of the total outstanding ICON ordinary shares. Although none of them has entered into any agreement obligating them to do so, ICON currently expects that all of its directors and executive officers will vote their shares “FOR” the ICON share issuance proposal and “FOR” the ICON adjournment proposal. See also the section entitled “Interests of ICON’s Directors and Executive Officers in the Merger” beginning on page 166 and the arrangements described in ICON’s Annual Report on Form 20-F filed with the SEC on February 24, 2021, which is incorporated into this joint proxy statement/prospectus by reference.

You may also cast your vote in person at ICON EGM. Even if you plan to attend the ICON EGM, ICON recommends that you vote your shares in advance as described above so that your vote will be counted if you later decide not to or become unable to attend the ICON EGM. In light of public health concerns related to COVID-19, ICON would like to emphasize that it considers the health of its shareholders, employees, attendees and other stakeholders a top priority and in this context is closely monitoring the evolving COVID-19 situation as regards the ICON EGM. Shareholders are referred to the section below entitled “Attending the ICON EGM” which sets out further detail on the situation in Ireland and restrictions around in person attendance at the ICON offices. ICON therefore strongly encourages voting via mail, telephone and Internet over in person attendance at the ICON EGM.

To be effective for ICON voting, the proxy card duly completed and executed, together with any authority under which it is executed, or a copy thereof certified, must be received at the registered office of ICON, so as to be received no later than 11:59 p.m. Eastern Time on Monday, June 14, 2021 or if the ICON EGM is adjourned, on the day that falls before the day appointed for the adjourned meeting.

Alternatively, provided it is received by 11:59 p.m., Eastern Time, on Sunday, June 13, 2021, or if the ICON EGM is adjourned, by 11:59 p.m., Eastern Time, on the day that falls 48 hours before the time appointed for the adjourned meeting, the appointment of a proxy may be submitted electronically, subject to the applicable terms and conditions, via the Internet or by telephone.

If your shares are held in “street name” by a bank, broker or other nominee, you should follow the instructions you receive from your bank, broker or other nominee on how to vote your shares. Registered shareholders who attend the ICON EGM may vote their shares personally even if they previously have voted their shares.

Any shareholder giving a proxy has the right to revoke it before the proxy is voted at the ICON EGM by taking any of the following actions:

Execution or revocation of a proxy will not in any way affect a shareholder’s right to attend the ICON EGM and vote in person.

If your shares are held in “street name” and you previously provided voting instructions to your broker, bank or other nominee, you should follow the instructions provided by your broker, bank or other nominee to revoke or change your voting instructions.

Unless revoked, all proxies representing shares entitled to vote that are delivered pursuant to this solicitation will be voted at the ICON EGM and, where a choice has been specified on the proxy card, will be voted in accordance with such specification. If an ICON shareholder makes no specification on his, her or its proxy card as to how such ICON shareholder should want their ICON ordinary shares voted, such proxy will be voted as recommended by the ICON board of directors as stated in this joint proxy statement/prospectus, specifically “FOR” the ICON share issuance proposal, and “FOR” the ICON adjournment proposal.

ICON is soliciting proxies to provide an opportunity to all ICON shareholders to vote on agenda items, whether or not the shareholders are able to attend the ICON EGM or an adjournment or postponement thereof. ICON will bear the entire cost of soliciting proxies from its stockholders. In addition to the solicitation of proxies by mail, ICON will ask banks, brokers and other custodians, nominees and fiduciaries to forward the proxy solicitation materials to the beneficial owners of ICON ordinary shares held of record by such nominee holders. ICON may be required to reimburse these nominee holders for their customary clerical and mailing expenses incurred in forwarding the proxy solicitation materials to the beneficial owners.

ICON has retained Georgeson LLC to assist in the solicitation process. ICON will pay Georgeson a fee of approximately $20,000, plus costs and expenses. ICON also has agreed to indemnify Georgeson against various liabilities and expenses that relate to or arise out of its solicitation of proxies (subject to certain exceptions). In addition to solicitation by mail, ICON’s directors, officers and other employees may solicit proxies in person, by telephone, electronically, by mail or other means. These persons will not be specifically compensated for doing this.

You are entitled to attend the ICON EGM only if you are a shareholder of record of ICON at the close of business on April 26, 2021 (the record date for the ICON EGM) or hold a proxy for such a shareholder. Shares held in “street name” may be voted in person by you only if you obtain a signed legal proxy from your bank, broker or other nominee giving you the right to vote the shares at the ICON EGM.

In light of public health concerns related to COVID-19, ICON would like to emphasize that it considers the health of its shareholders, employees, attendees and other stakeholders a top priority, and in this context, is closely monitoring the evolving COVID-19 situation.

Based on the latest available public health guidance, it is expected the ICON EGM will proceed under very constrained circumstances given current restrictions on public gatherings in Ireland. Shareholders are strongly encouraged not to attend the meeting in person and instead to vote their shares by proxy as the preferred method of fully and safely exercising their rights. Personal attendance at the AGM may present a health risk to shareholders and others and entry may be refused. ICON advises that shareholders who are experiencing any COVID-19 symptoms or anyone who has been in contact with any person experiencing any COVID-19 symptoms should not attend the ICON EGM in person.

ICON may take additional procedures or limitations applicable to meeting attendees, including restrictions on seating arrangements, health screening requirements and other reasonable or required measures in order to enter the building.

In the event that a change of venue becomes necessary due to public health recommendations regarding containment of COVID-19, which may include the closure of or restrictions on access to the meeting venue, ICON will promptly communicate this to shareholders by an announcement in a press release, on the investor relations page of https://investor.iconplc.com/ and a filing with the U.S. Securities and Exchange Commission. ICON advises shareholders to monitor the page regularly, as circumstances may change on short notice. We recommend that shareholders keep up-to-date with the latest public health guidance regarding travel, self-isolation and health and safety precautions.

The preliminary voting results will be announced at the ICON EGM. In addition, within four (4) business days following the ICON EGM, ICON intends to file the final voting results with the SEC on a Form 6-K. If the final voting results have not been certified within that four-business-day period, ICON will report the preliminary voting results on a Form 6-K at that time and will file an amendment to the Form 6-K to report the final voting results within four (4) days of the date that the final results are certified.

If a quorum is present at the ICON EGM but there are not sufficient votes at the time of the ICON EGM to approve the ICON share issuance proposal, then ICON shareholders may be asked to vote on the ICON adjournment proposal.

If the adjournment is for more than fourteen (14) days, ICON will give at least seven (7) days clear notice of the adjourned meeting to each ICON shareholder of record entitled to vote at the adjourned meeting as of the record date for determining the stockholders entitled to notice of the ICON EGM.

At any subsequent reconvening of the ICON EGM at which a quorum is present, any business may be transacted that might have been transacted at the original meeting and all proxies will be voted in the same manner as they would have been voted at the original convening of the ICON EGM, except for any proxies that have been effectively revoked or withdrawn prior to the time the proxy is voted at the reconvened meeting.

If you need assistance voting or in completing your proxy card or have questions regarding the ICON EGM, please contact Georgeson LLC, the proxy solicitation agent for ICON:

This joint proxy statement/prospectus is being furnished to you as a shareholder of ICON as part of the solicitation of proxies by the ICON board of directors for use at the ICON EGM to consider and vote upon a proposal to approve the issuance of ICON ordinary shares to PRA stockholders pursuant to the merger agreement, which is attached as Annex A to this joint proxy statement/prospectus.

Under Irish law, the directors of a company may issue new shares without shareholder approval once authorized to do so by its constitution or by an ordinary resolution adopted by the shareholders at a general meeting. ICON has a pre-existing shareholder approval, from its last annual general meeting, to issue shares up to an amount equal to approximately 20% of its issued and outstanding share capital. It is currently estimated that ICON will issue or reserve for issuance approximately 28,831,766 ICON ordinary shares to PRA stockholders pursuant to the merger agreement, which will exceed the existing 20% approval limit. Accordingly ICON shareholder approval is required prior to the issuance of ICON ordinary shares to PRA stockholders pursuant to the merger agreement.

Approval of the share issuance is a condition to the completion of the merger.

The ICON board of directors unanimously recommends that ICON shareholders approve the following resolution as an ordinary resolution:

“That, subject to applicable rules and listing standards of Nasdaq, the Directors of ICON be and they are hereby generally and unconditionally authorized pursuant to section 1021 of the Companies Act 2014 to exercise all the powers of ICON to allot relevant securities (within the meaning of section 1021 of the Companies Act 2014) as contemplated by the Agreement and Plan of Merger, dated as of February 24, 2021 (as it may be amended from time to time), which is referred to as the merger agreement, by and among ICON, ICON US Holdings Inc., Indigo Merger Sub, Inc. and PRA Health Sciences, Inc., up to an aggregate nominal amount of ICON ordinary shares necessary for purposes of satisfying the aggregate issuance of ICON ordinary shares in connection with and pursuant to the merger agreement, provided that such authority shall (a) expire on December 31, 2022 or such later date as may be determined by the ICON board of directors from time to time (provided that under the Companies Act, such later date cannot be more than five (5) years after the date on which this resolution is passed), and (b) be without prejudice and in addition to the authority under the said Section 1021 previously granted to the ICON board of directors pursuant to an ordinary resolution passed at the annual general meeting of ICON held on July 21, 2020.”

The ICON share issuance proposal is being proposed as an ordinary resolution and therefore requires the affirmative vote of a majority of votes cast, in person or by proxy, on the proposal. A shareholder’s abstention and a broker non-vote or other failure to vote will have no effect on the proposal.

The ICON EGM may be adjourned to another time and place if necessary to permit solicitation of additional proxies if there are not sufficient votes to approve the ICON share issuance proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to the ICON shareholders.

ICON is asking its shareholders to authorize the holder of any proxy solicited by the ICON board of directors to vote in favor of any such adjournment to the ICON EGM, if such an adjournment is required or deemed appropriate.

The ICON board of directors unanimously recommends that ICON shareholders approve the following resolution as an ordinary resolution:

“That the Chairperson of the meeting be and is hereby authorized to adjourn the meeting to another time and place if, in the discretion of the Chairperson, it is necessary or appropriate to, among other things, solicit additional proxies if there are insufficient votes received by way of proxy, at the time of the meeting to approve the ICON share issuance proposal.”

The ICON adjournment proposal is being proposed as an ordinary resolution and therefore requires the affirmative vote of a majority of votes cast, in person or by proxy, on the proposal. A shareholder’s abstention and a broker non-vote or other failure to vote will have no effect on the proposal.

This joint proxy statement/prospectus is being mailed on or about April 29, 2021, to holders of record of PRA common stock as of the close of business on April 26, 2021, and constitutes notice of the PRA stockholder meeting in conformity with the requirements of the DGCL.

This joint proxy statement/prospectus is being provided to PRA stockholders as part of a solicitation of proxies by the PRA board of directors for use at the PRA stockholder meeting and at any adjournments or postponements of the PRA stockholder meeting. PRA stockholders are encouraged to read the entire document carefully, including the annexes to and documents incorporated by reference into this document, for more detailed information regarding the merger agreement and the transactions contemplated by the merger agreement.

The live, audio-only virtual PRA stockholder meeting is scheduled to be held at 10:30 a.m., Eastern Time, on June 15, 2021, at www.virtualshareholdermeeting.com/PRAH2021SM, unless postponed to a later date. The PRA stockholder meeting will be held online only and you will not be able to attend in person. Online check-in will begin at 10:15 a.m. Eastern Time and you should allow ample time for the check-in procedures. You will be able to vote your shares electronically by Internet and submit questions online during the PRA stockholder meeting by logging in to the website listed above using the 16-digit control number included in your proxy card.

The purposes of the PRA stockholder meeting are as follows, each as further described in this joint proxy statement/prospectus:

The PRA board of directors unanimously recommends that PRA stockholders vote:

After careful consideration, PRA’s board of directors unanimously (i) determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement are fair to, and in the best interests of, PRA and its stockholders and (ii) approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement.

See also the section entitled “The Merger—Recommendation of the PRA Board of Directors; PRA’s Reasons for the Merger” beginning on page 73.

The record date to determine who is entitled to receive notice of and to vote at the PRA stockholder meeting or any adjournments or postponements thereof is April 26, 2021. As of the close of business on the record date, there were 64,795,400 shares of PRA common stock outstanding and entitled to vote at the PRA stockholder meeting. Each PRA stockholder is entitled to one vote for any matter properly brought before the PRA stockholder meeting for each share of PRA common stock such holder owned at the close of business on the record date. Only PRA stockholders of record at the close of business on the record date are entitled to receive notice of and to vote at the PRA stockholder meeting and any and all adjournments or postponements thereof. In addition, the stockholder list will be available for inspection during the PRA stockholder meeting at www.virtualshareholdermeeting.com/PRAH2021SM.

A quorum of stockholders is necessary to conduct the PRA stockholder meeting. A quorum exists if the holders of at least a majority of the issued and outstanding shares of PRA common stock entitled to vote on such subject matter are represented in person or by proxy at such meeting. PRA stockholders who virtually attend the PRA stockholder meeting via live audio-only webcast at www.virtualshareholdermeeting.com/PRAH2021SM will be considered present “in person” for purposes of establishing a quorum and for all other purposes. Shares of PRA common stock represented at the PRA stockholder meeting and entitled to vote, but not voted, including shares for which a stockholder directs an “abstention” from voting and broker non-votes will be counted for purposes of determining a quorum. If a quorum is not present, the PRA stockholder meeting will be postponed until the holders of the number of shares of PRA common stock required to constitute a quorum attend.

Under stock exchange rules, banks, brokers or other nominees who hold shares in “street name” for a beneficial owner of those shares typically have the authority to vote in their discretion on “routine” proposals when they have not received instructions from beneficial owners. However, banks, brokers or other nominees are not allowed to exercise their voting discretion with respect to the approval of matters that are “non-routine.” Generally, a broker non-vote occurs on an item when (i) a bank, broker or other nominee has discretionary authority to vote on one or more “routine” proposals to be voted on at a meeting of stockholders, but is not permitted to vote on other “non-routine” proposals without instructions from the beneficial owner of the shares and (ii) the beneficial owner fails to provide the bank, broker or other nominee with such instructions. Under stock exchange rules, “non-routine” matters include the PRA merger agreement proposal (PRA Proposal 1), the PRA non-binding advisory compensation proposal (PRA Proposal 2) and the PRA adjournment proposal (PRA Proposal 3). Because none of the proposals to be voted on at the PRA stockholder meeting are routine matters for which brokers may have discretionary authority to vote, PRA does not expect any broker non-votes at the PRA stockholder meeting. As a result, if you hold your shares of PRA common stock in “street name,” your shares will not be represented and will not be voted on any matter unless you affirmatively instruct your bank, broker or other nominee how to vote your shares in one of the ways indicated by your bank, broker or other nominee. It is therefore critical that you cast your vote by instructing your bank, broker or other nominee on how to vote.

If you submit a properly executed proxy card, even if you abstain from voting or vote against the approval of the share issuance or the adoption of the charter amendment, your shares of PRA common stock will be counted for purposes of calculating whether a quorum is present at the PRA stockholder meeting. Executed but unvoted proxies will be voted in accordance with the recommendations of the PRA board of directors. If additional votes must be solicited to approve the share issuance or adopt the charter amendment, it is expected that the meeting will be adjourned to solicit additional proxies.

Except for the PRA adjournment proposal, the vote required to approve all of the proposals listed herein assumes the presence of a quorum.

As of the record date, PRA directors and executive officers, and their affiliates, as a group, owned and were entitled to vote 981,275 shares of PRA common stock, or approximately 1.5% of the total outstanding shares of PRA common stock. PRA currently expects that all of its directors and executive officers will vote their shares “FOR” the PRA merger agreement proposal, “FOR” the PRA compensation proposal and “FOR” the PRA adjournment proposal. See also the section entitled “Interests of PRA’s Directors and Executive Officers in the Merger” beginning on page 167 and PRA’s Amendment to Annual Report on Form 10-K/A for the fiscal year ended December 31, 2020 filed with the SEC on March 30, 2021, which is incorporated into this joint proxy statement/prospectus by reference.

If you are a stockholder of record, proxies submitted over the Internet, by telephone or by mail as described above must be received by 11:59 p.m., Eastern Time, on June 14, 2021.

Notwithstanding the above, if you hold your shares in “street name” and you submit voting instructions to your bank, broker or other nominee, your instructions must be received by the bank, broker or other nominee prior to the deadline set forth in the information from your bank, broker or other nominee on how to submit voting instructions.

If you deliver a proxy pursuant to this joint proxy statement/prospectus, but do not specify a choice with respect to any proposal set forth in this joint proxy statement/prospectus, your underlying shares of PRA common stock will be voted on such uninstructed proposal in accordance with the recommendation of the PRA board of directors. At the date hereof, PRA’s management has no knowledge of any business that will be presented for consideration at the PRA stockholder meeting and which would be required to be set forth in this joint proxy statement/prospectus or the related PRA proxy card other than the matters set forth in PRA’s Notice of Special Meeting of Stockholders. If any other matter is properly presented at the PRA stockholder meeting for consideration, it is intended that the persons named in the enclosed form of proxy and acting thereunder will vote in accordance with their best judgment on such matter.

Any stockholder giving a proxy has the right to revoke it before the proxy is voted at the PRA stockholder meeting by any of the following actions:

Execution or revocation of a proxy will not in any way affect the stockholder’s right to attend and vote at the virtual stockholder meeting.

Written notices of revocation and other communications with respect to the revocation of proxies should be addressed to:

If your shares are held in “street name” and you previously provided voting instructions to your broker, bank or other nominee, you should follow the instructions provided by your broker, bank or other nominee to revoke or change your voting instructions.

Unless revoked, all proxies representing shares entitled to vote that are delivered pursuant to this solicitation will be voted at the PRA stockholder meeting and, where a choice has been specified on the proxy card, will be voted in accordance with such specification. If a PRA stockholder makes no specification on his, her or its proxy card as to how such PRA stockholder should want his, her or its shares of PRA common stock voted, such proxy will be voted as recommended by the PRA board of directors as stated in this joint proxy statement/prospectus, specifically “FOR” the PRA merger agreement proposal, “FOR” the PRA compensation proposal and “FOR” the PRA adjournment proposal.

PRA is soliciting proxies to provide an opportunity to all PRA stockholders to vote on agenda items, whether or not the stockholders are able to attend the PRA stockholder meeting or an adjournment or postponement thereof.

PRA will bear the entire cost of soliciting proxies from its stockholders. In addition to the solicitation of proxies by mail, PRA will request that banks, brokers and other nominee record holders send proxies and proxy material to the beneficial owners of PRA common stock and secure their voting instructions, if necessary. PRA may be required to reimburse those banks, brokers and other nominees on request for their reasonable expenses in taking those actions.

PRA has also made arrangements with MacKenzie to assist in soliciting proxies and in communicating with PRA stockholders and estimates that it will pay them a fee of approximately $50,000 plus reimbursement for certain fees and expenses. PRA also has agreed to indemnify MacKenzie against various liabilities and expenses that relate to or arise out of its solicitation of proxies (subject to certain exceptions). Proxies may be solicited on behalf of PRA in person, by mail, by telephone, by facsimile, by messenger, via the Internet or by other means of communication, including electronic communication, or by PRA directors, officers and other employees in person, by mail, by telephone, by facsimile, via the Internet or by other means of communication, including electronic communication. Directors, officers and employees of PRA will not be specially compensated for their services or solicitation in this regard.

The PRA stockholder meeting will be held entirely online due to the public health concerns regarding the coronavirus (COVID-19) outbreak. You will not be able to attend the PRA stockholder meeting in person. The meeting will be held virtually on June 15, 2021 at 10:30 a.m. Eastern Time via live audio-only webcast at www.virtualshareholdermeeting.com/PRAH2021SM. To attend the meeting as a stockholder, you will need the 16-digit control number included in your proxy card. Online check-in will begin at 10:15 a.m. Eastern Time and you should allow ample time for the check-in procedures. If you do not have your 16-digit control number, you will be able to access and listen to the PRA stockholder meeting but you will not be able to vote your shares or submit questions during the PRA stockholder meeting.

The virtual meeting has been designed to provide the same rights to participate as the stockholder would have at an in-person meeting. Information on how to vote by Internet before and during the PRA stockholder meeting is discussed above.

If you plan to virtually attend the PRA stockholder meeting and vote, PRA still encourages you to vote in advance by the Internet, telephone or (if you received a paper copy of the proxy materials) by mail so that your vote will be counted even if you later decide not to attend the PRA stockholder meeting. Voting your proxy by the Internet, telephone or mail will not limit your right to vote at the PRA stockholder meeting if you later decide to virtually attend. If you own your shares of PRA common stock in “street name” and wish to vote at the PRA stockholder meeting, you must obtain a signed legal proxy from your bank, broker or other nominee giving you the right to vote the shares.

The virtual PRA stockholder meeting allows PRA stockholders to submit questions during the PRA stockholder meeting in the question box provided at www.virtualshareholdermeeting.com/PRAH2021SM. PRA will respond to as many inquiries at the PRA stockholder meeting as time allows.

PRA will have technicians ready to assist you with any technical difficulties you may have accessing the virtual meeting website. If you encounter any difficulties accessing the virtual meeting website during the check-in or meeting time, please call the technical support number that will be posted on the virtual meeting website log-in page at www.virtualshareholdermeeting.com/PRAH2021SM.

You may vote your shares before the PRA stockholder meeting by Internet, by proxy or by telephone pursuant to the instructions contained in your proxy card. You do not need to access the PRA stockholder meeting webcast to vote if you submitted your vote via proxy, by Internet or by telephone in advance of the PRA stockholder meeting.

SEC rules permit companies and intermediaries such as brokers to satisfy delivery requirements for proxy statements and notices with respect to two (2) or more stockholders sharing the same address by delivering a single proxy statement or a single notice addressed to those stockholders. This process, which is commonly referred to as

“householding,” provides cost savings for companies. Some brokers household proxy materials, delivering a single proxy statement or notice to multiple stockholders sharing an address unless contrary instructions have been received from the affected stockholders. Once you have received notice from your broker that they will be householding materials to your address, householding will continue until you are notified otherwise or until you revoke your consent. If, at any time, you no longer wish to participate in householding and would prefer to receive a separate proxy statement or notice, or if your household is receiving multiple copies of these documents and you wish to request that future deliveries be limited to a single copy, please notify your broker. You can request prompt delivery of a copy of this joint proxy statement/prospectus by writing to: Corporate Secretary, PRA Health Sciences, Inc., 130 ParkLake Avenue, Suite 400, Raleigh, North Carolina 27612 or by calling 1-919-786-8200.

Representatives of Broadridge Financial Solutions, Inc. will tabulate the votes and will act as independent inspector of election at the PRA stockholder meeting.

Preliminary voting results will be announced at the PRA stockholder meeting. In addition, PRA intends to file the final voting results with the SEC on a Current Report on Form 8-K within four (4) business days after the PRA stockholder meeting.

The chairperson of the meeting or a majority in voting power of the shares represented at the meeting may adjourn the meeting if no quorum is present.

If a quorum is present at the PRA stockholder meeting but there are not sufficient votes at the time of the PRA stockholder meeting to approve the PRA merger agreement proposal, then PRA stockholders may be asked to vote on the PRA adjournment proposal.

If the adjournment is for more than 30 days, PRA will give notice of the adjourned meeting to each PRA stockholder of record entitled to vote at the PRA stockholder meeting. If, after the adjournment, a new record date for determination of stockholders entitled to vote is fixed for the adjourned PRA stockholder meeting, the PRA board of directors will fix as the record date for determining PRA stockholders entitled to notice of such adjourned PRA stockholder meeting the same or an earlier date as that fixed for determination of PRA stockholders entitled to vote at the adjourned PRA stockholder meeting, and will give notice of the adjourned PRA stockholder meeting to each PRA stockholder of record entitled to vote at such adjourned meeting PRA stockholder meeting as of the record date so fixed for notice of such adjourned PRA stockholder meeting.

At any subsequent reconvening of the PRA stockholder meeting at which a quorum is present, any business may be transacted that might have been transacted at the original meeting and all proxies will be voted in the same manner as they would have been voted at the original convening of the PRA stockholder meeting, except for any proxies that have been effectively revoked or withdrawn prior to the time the proxy is voted at the reconvened meeting.

If you need assistance voting or in completing your proxy card or have questions regarding the PRA stockholder meeting, please contact MacKenzie Partners, Inc., the proxy solicitation agent for PRA:

This joint proxy statement/prospectus is being furnished to you as a stockholder of PRA as part of the solicitation of proxies by the PRA board of directors for use at the PRA stockholder meeting to consider and vote upon a proposal to adopt the merger agreement, which is attached as Annex A to this joint proxy statement/prospectus.

The PRA board of directors, after due and careful discussion and consideration, unanimously approved and declared advisable the merger agreement, the merger and the other transactions contemplated by the merger agreement and determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement are fair to and in the best interests of PRA and its stockholders.

The PRA board of directors accordingly unanimously recommends that PRA stockholders adopt the merger agreement, as disclosed in this joint proxy statement/prospectus and particularly the related narrative disclosures in the sections of this joint proxy statement/prospectus entitled “The Merger” beginning on page 66 and “The Merger Agreement” beginning on page 116 and as attached as Annex A to this joint proxy statement/prospectus.

The merger between Merger Sub and PRA cannot be completed without the affirmative vote of a majority of the outstanding shares of PRA common stock entitled to vote thereon. A failure to vote, a broker non-vote or an abstention will have the same effect as a vote “AGAINST” the proposal to adopt the merger agreement.

Pursuant to Section 14A of the Exchange Act and the applicable SEC rules issued thereunder, PRA is seeking a non-binding, advisory stockholder approval of the compensation of PRA’s named executive officers that is based on or otherwise relates to the merger as disclosed in the section entitled “Interests of PRA’s Directors and Executive Officers in the Merger—Merger-Related Compensation—Golden Parachute Compensation” beginning on page 170.

Approval of the advisory compensation proposal is not a condition to completion of the merger and is a vote separate and apart from the vote to adopt the merger agreement. Accordingly, if you are a PRA stockholder, you may vote to approve the PRA merger agreement proposal, and vote not to approve the PRA compensation proposal, and vice versa. If the merger is completed, the merger-related compensation may be paid to PRA’s named executive officers to the extent payable in accordance with the terms of the compensation agreements and arrangements even if PRA stockholders fail to approve the advisory vote regarding merger-related compensation. However, PRA seeks the support of its stockholders and believes that stockholder support is appropriate as the executive compensation programs are designed to incentivize executives to successfully execute a transaction such as that contemplated by the merger proposal from its early stages until consummation.

Accordingly, PRA is asking PRA stockholders to vote “FOR” the adoption of the following resolution, on a non-binding, advisory basis:

“RESOLVED, that the compensation that will or may be paid or become payable to PRA’s named executive officers that is based on or otherwise relates to the merger, and the agreements or understandings pursuant to which such compensation will or may be paid or become payable, in each case as disclosed pursuant to Item 402(t) of Regulation S-K in “Interests of PRA’s Directors and Executive Officers in the Merger—Merger-Related Compensation—Golden Parachute Compensation” is hereby APPROVED.”

Approval of the PRA compensation proposal requires the affirmative vote of a majority in voting power of the shares of PRA common stock represented at the PRA stockholder meeting. A stockholder’s abstention from voting will have the same effect as a vote “AGAINST” the PRA compensation proposal, while a broker non-vote or other failure to vote (including a failure to instruct your bank, broker or other nominee to vote) will have no effect on the outcome of the proposal.

The PRA stockholder meeting may be adjourned to another time and place if necessary to permit solicitation of additional proxies if there are not sufficient votes to approve the PRA merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to the PRA stockholders.

PRA is asking its stockholders to authorize the holder of any proxy solicited by the PRA board of directors to vote in favor of any adjournment of the PRA stockholder meeting to solicit additional proxies if there are not sufficient votes to approve the PRA merger agreement proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to PRA stockholders.

The PRA board of directors unanimously recommends that PRA stockholders approve the proposal to adjourn the PRA stockholder meeting, if necessary.

Whether or not there is a quorum, approval of the PRA adjournment proposal requires the affirmative vote of a majority in voting power of the shares of PRA common stock represented at the PRA stockholder meeting. A stockholder’s abstention from voting will have the same effect as a vote “AGAINST” the PRA adjournment proposal, while a broker non-vote or other failure to vote (including a failure to instruct your bank, broker or other nominee to vote) will have no effect on the outcome of the proposal.

Under the PRA bylaws, the chairperson of the PRA stockholder meeting may adjourn the PRA stockholder meeting if no quorum is present.

The following is a description of the material aspects of the merger. While ICON and PRA believe that the following description covers the material terms of the merger, the description may not contain all of the information that is important to you. You are encouraged to read carefully this entire joint proxy statement/prospectus, including the text of the merger agreement attached to this joint proxy statement/prospectus as Annex A, for a more complete understanding of the merger. In addition, important business and financial information about each of ICON and PRA is included in or incorporated by reference into this joint proxy statement/prospectus. See “Where You Can Find More Information” beginning on page 218.

ICON and PRA have entered into the merger agreement, which provides for the merger of Merger Sub, a Delaware corporation and a wholly owned subsidiary of ICON and US HoldCo, with and into PRA. As a result of the merger, the separate existence of Merger Sub will cease, PRA will continue as the surviving corporation and PRA will be a wholly owned subsidiary US HoldCo and ICON.

At the effective time, by virtue of the merger and without any action on the part of the parties or any holder of any capital stock of PRA, each share of PRA common stock issued and outstanding immediately prior to the effective time (other than excluded shares, which refers to shares of PRA common stock owned, prior to the effective time, by ICON, US HoldCo, Merger Sub or any other direct or indirect wholly owned subsidiary of ICON or any direct or indirect wholly owned subsidiary of PRA, or held in PRA’s treasury, and dissenting shares (as defined in the section entitled “The Merger Agreement—Dissenting Shares” beginning on page 119)) will be converted automatically into the right to receive (i) 0.4125 of one ICON ordinary share and (ii) $80.00 in cash, without interest.

No fractional ICON ordinary shares will be issued upon the conversion of shares of PRA common stock pursuant to the merger agreement. Each holder of shares of PRA common stock who would otherwise have been entitled to receive fractional ICON ordinary shares pursuant to the merger agreement (after taking into account all shares of PRA common stock exchanged by such holder) will be entitled to receive a cash payment, without interest, in lieu of any such fractional share, equal to the product of (i) such fractional amount and (ii) an amount equal to the last reported sale price per share of ICON ordinary shares on Nasdaq (as reported in The Wall Street Journal or an authoritative source mutually agreed in good faith by ICON and PRA) on the last complete trading day immediately prior to the date of the effective time.

The exchange ratio is fixed, which means that it will not change between now and the date of the merger, regardless of whether the market price of either ICON ordinary shares or PRA common stock changes. The market price of ICON ordinary shares has fluctuated since the date of the announcement of the merger agreement and will continue to fluctuate from the date of this joint proxy statement/prospectus to the date of the stockholder meetings and the date the merger is completed and thereafter. The market price of ICON ordinary shares, when received by PRA stockholders after the merger is completed, could be greater than, less than or the same as the market price of ICON ordinary shares on the date of this joint proxy statement/prospectus or at the time of the stockholder meetings. Accordingly, you should obtain current stock price quotations for ICON ordinary shares and PRA common stock before deciding how to vote with respect to the proposals described in this joint proxy statement/prospectus. The common stock of ICON is traded on Nasdaq under the symbol “ICLR” and the common stock of PRA is traded on Nasdaq under the symbol “PRAH.”

At the effective time, all excluded shares other than subsidiary-held shares will be cancelled and will cease to exist, and no payment will be made in respect of such shares.

The terms of the merger agreement are the result of arm’s-length negotiations between PRA and ICON. The following is a summary of the material events leading up to the entry into the merger agreement and the key meetings, negotiations, discussions and actions between PRA and ICON and their respective advisors that preceded the public announcement of the merger.

The PRA board of directors and PRA management periodically review the strategic direction of PRA and evaluate potential opportunities to enhance stockholder value, including through potential business combination

transactions. As part of these reviews, the PRA board of directors has received periodic updates from management and its advisors regarding the clinical research organization (“CRO”) industry, including the continuing consolidation in the CRO industry, PRA’s positioning relative to its peers, capital allocation strategies, and strategic alternatives.

From time to time in the ordinary course of business, senior executives of PRA, including Colin Shannon, the President and Chief Executive Officer of PRA, have had informal and preliminary conversations about potential strategic combinations in the CRO industry with senior executives of other companies in the CRO industry (including at industry conferences and meetings of industry executives). PRA senior management updates the PRA board of directors regarding these interactions during regularly scheduled and special meetings of the PRA board of directors.

On January 29, 2020, Mr. Shannon and Dr. Cutler had a telephone conversation. During the conversation, they discussed the state of the CRO industry, including the continuing consolidation in the CRO industry. No details were discussed with respect to a potential business combination.

On February 24, 2020, Mr. Shannon and Dr. Cutler had a telephone conversation to discuss the state of the CRO industry. No details were discussed with respect to a potential business combination, but Mr. Shannon and Dr. Cutler agreed to speak again regarding a potential business combination and agreed that the two companies should enter into a mutual confidentiality agreement that would enable them to continue their discussions. Following the conversation, Mr. Shannon informed members of each of the PRA board of directors and senior management team regarding what was discussed, including that PRA and ICON would enter into a mutual confidentiality agreement on mutually satisfactory terms.

On February 28, 2020, in anticipation of the next meeting between Mr. Shannon and Dr. Cutler, PRA and ICON entered into a mutual confidentiality agreement. The confidentiality agreement did not contain a standstill provision limiting ICON’s ability to acquire any of PRA’s securities, to participate in any proxy solicitation for PRA stockholder votes or to take similar actions.

Mr. Shannon and Dr. Cutler agreed to meet on March 17, 2020 to continue discussing a potential business combination, but the meeting was canceled due to the onset of the COVID-19 pandemic.

Between mid-March and October, 2020 the companies did not engage in substantive discussions regarding a potential transaction.

On October 1, 2020, the meeting between Mr. Shannon and Dr. Cutler, which had been rescheduled from its earlier date, took place. During the meeting, Mr. Shannon and Dr. Cutler discussed the state of the CRO industry and the possibility of a combination of the two companies. Mr. Shannon and Dr. Cutler did not make any specific proposals during the meeting.

On October 9, 2020, during a telephone conversation with Mr. Shannon, Dr. Cutler informed Mr. Shannon that he would review the opportunity to pursue a potential business combination with the ICON board of directors during its regularly scheduled meeting the week of October 20 and, if he received support from the ICON board of directors, would respond to Mr. Shannon the week of October 26 to begin coordinating the sharing of high level information that would allow ICON to validate the rationale for a potential business combination and formulate a more specific proposal.

Mr. Shannon subsequently informed members of each of the PRA board of directors and senior management team of the foregoing conversation.

On October 15, 2020, following preliminary discussions unrelated to the potential transaction with ICON, PRA entered into a non-binding letter of intent with Company A, which set forth preliminary terms of a transaction pursuant to which Company A would acquire a PRA business unit. Following the execution of the letter of intent, Company A conducted more extensive due diligence regarding the business unit. However, no definitive agreements were executed with Company A regarding this potential transaction.

On October 26, 2020, Dr. Cutler informed Mr. Shannon during a telephone conversation that the ICON board of directors had approved moving forward with an information gathering session and that Dr. Cutler would brief the ICON board of directors at their annual December strategy session and make a recommendation on whether to move forward with a potential transaction with PRA. Mr. Shannon and Dr. Cutler agreed to hold a high level meeting to share information on their respective businesses before December.

On November 17, 2020, at a regularly scheduled meeting of the PRA board of directors, Mr. Shannon updated the PRA board of directors on the status of discussions with each of Dr. Cutler and with Company A. The PRA board of directors determined that it was supportive of further discussions with ICON for a number of reasons, including the trend towards consolidation in the CRO industry, the benefits of bringing together PRA’s geographic and biotechnology focus with ICON’s geographic and pharmaceutical focus and the suite of capabilities that the combined company could provide to clients, and authorized PRA senior management to have such discussions and exchange confidential information with ICON in furtherance of a potential business combination transaction, but instructed PRA senior management not to discuss any matters pertaining to post-closing employment or compensation, or provide any compensation information to ICON.

On November 20, 2020, members of each of the senior management teams of ICON and PRA met by video conference. At the meeting, PRA management presented an overview of PRA’s business, and ICON management presented an overview of ICON’s business.

On December 10, 2020, following a meeting of the ICON board of directors, Dr. Cutler informed Mr. Shannon by telephone that the ICON board of directors was enthusiastic about a potential business combination with PRA and proposed executing a non-binding letter of intent by mid-January, with the goal of conducting due diligence in January and February, 2021, signing a definitive merger agreement by February 23 or 24 and closing the proposed transaction at the end of the second quarter of 2021. Dr. Cutler noted that any ICON proposal would be conditioned on PRA agreeing to grant ICON exclusivity.

Mr. Shannon subsequently informed members of each of the PRA board of directors and senior management team of the foregoing conversation.

On January 7, 2021, ICON delivered to PRA a written non-binding indication of interest in a transaction in which (i) each share of PRA common stock would be exchanged for $150, comprised of $75 in cash and a number of ICON ordinary shares representing a value of $75 based on a fixed exchange ratio determined by the 20-trading day VWAP of ICON ordinary shares ending the day prior to signing and (ii) Mr. Shannon would join the combined company board of directors. This indication of interest is referred to as the ICON January 7, 2021 proposal. The ICON January 7, 2021 proposal was conditioned on PRA entering into an exclusivity agreement, a draft of which was delivered simultaneously with the ICON January 7, 2021 proposal, and contemplated a target announcement date of February 24, 2021. The draft exclusivity agreement provided for exclusivity through February 28, 2021.

On January 19, 2021, the PRA board of directors held a special meeting. Members of PRA’s senior management team and representatives of each of BofA Securities, UBS and PRA’s outside legal counsel also attended the meeting. Members of PRA’s senior management team updated the PRA board of directors on the status of discussions with Company A, noting that very little progress had been made due to fundamental differences with respect to key commercial and other material terms. Representatives of PRA’s outside legal counsel then reviewed with the PRA board of directors its fiduciary duties under applicable law with respect to its consideration and exploration of a potential transaction with ICON, and certain material relationships of BofA Securities and UBS with PRA and ICON. Members of PRA’s senior management reviewed with the PRA board of directors a forecast of PRA’s future performance, which PRA management proposed to be used by BofA Securities and UBS for purposes of their respective financial analyses and opinions and to be shared with ICON if discussions with ICON progressed. The PRA board of directors approved the financial forecast for such uses. For more information regarding such financial forecast, see the section entitled “The Merger—PRA Unaudited Financial Forecasts” beginning on page 109. Representatives of each of BofA Securities and UBS then joined the meeting and reviewed with the PRA board of directors the terms of the ICON January 7, 2021 proposal. The investment banks were engaged as financial advisors to PRA on January 20, 2021.

On January 21, 2021, Company A informed PRA that it was terminating discussions regarding a potential transaction.

Also on January 21, the PRA board of directors held a special meeting. Members of PRA’s senior management team and representatives of each of BofA Securities, UBS and PRA’s outside legal counsel also attended the meeting. Members of PRA’s senior management team updated the PRA board of directors on Company A’s termination of discussions regarding a potential transaction over fundamental differences regarding core commercial and other material terms. The representatives of each of BofA Securities and UBS then reviewed with the board their preliminary perspectives with respect to the ICON January 7, 2021 proposal. Further, the BofA Securities and UBS representatives reviewed with the PRA board of directors certain potential alternative strategic opportunities to

consider, including potential alternative business combination partners, and the PRA board of directors considered, following such review, the strategic rationale for business combinations with such partners and the strategic rationale for a business combination with ICON, including that a business combination with ICON would bring together two highly complementary businesses, that the combined company would be a top pure-play CRO and be well positioned in the event of future consolidation in the CRO industry, that the combined company could provide a comprehensive suite of services to support clients’ global drug development efforts and that the combination would bring together two highly regarded CROs that would be combining from positions of strength. The PRA board of directors also considered the risks associated with initiating a market check and the PRA board of directors’ view that a business combination with ICON was a unique strategic opportunity that could not be replicated through a transaction with another party and that ICON was best positioned to deliver a proposal offering the highest potential value to PRA stockholders. Further, the PRA board of directors discussed its view that consolidation in the industry was likely to happen, and that based on their understanding of the CRO industry, other participants in the industry either could not offer the price ICON would be able to offer because of financial constraints, did not have the right mix of business in their view to be as successful going forward or did not have stock to offer as consideration that was as attractive as the ICON ordinary shares through which PRA stockholders could participate in the growth of the combined company. The PRA board of directors authorized BofA Securities and UBS to reach out to ICON’s financial advisor, Centerview, and to indicate that the PRA board of directors understood the strategic rationale in a combination of PRA and ICON, however, the terms in the ICON January 7, 2021 proposal were insufficient and that in order for the PRA board of directors to consider moving forward on the timeline ICON proposed and potentially grant exclusivity, ICON would need to materially improve its proposal to address both the fundamental value of PRA’s business and to induce the PRA board of directors to move forward on a pre-emptive basis.

On January 22, 2021, as directed by the PRA board of directors, representatives of each of BofA Securities and UBS held a telephone conversation with representatives of Centerview during which they provided feedback on the ICON January 7, 2021 proposal consistent with the PRA board of directors’ authorization.

On January 23, 2021, during a telephone conversation among representatives of each of BofA Securities and UBS, representatives of Centerview indicated that ICON had revised its proposal to $160 per share of PRA common stock, comprised of $80 in cash and a number of ICON ordinary shares representing a value of $80 based on a fixed exchange ratio determined by the 20-trading day VWAP of ICON ordinary shares ending the day prior to signing. The Centerview representatives reiterated that ICON envisioned Mr. Shannon joining the combined company board of directors. This proposal is referred to as the ICON January 23, 2021 proposal. The Centerview representatives further indicated that the ICON January 23, 2021 proposal and ICON’s willingness to proceed was conditioned on PRA agreeing to the proposed exclusivity agreement.

That afternoon, the PRA board of directors held a special meeting. A representative of PRA’s outside legal counsel was present. The PRA board of directors discussed the ICON January 23, 2021 proposal and authorized BofA Securities and UBS to convey to Centerview that further price improvement would be necessary and to deliver to Centerview PRA’s fourth quarter 2020 financial results and financial forecast so that ICON could better understand the fundamental value of PRA.

That evening, as directed by the PRA board of directors, representatives of each of BofA Securities and UBS held a telephone conversation with representatives of Centerview during which they provided feedback on the ICON January 23, 2021 proposal consistent with the PRA board of directors’ authorization. Following the conversation, representatives of BofA Securities and UBS sent Centerview PRA’s fourth quarter 2020 financial results and financial forecast.

On January 27, 2021, Dr. Cutler called Mr. Shannon regarding the ICON January 23, 2021 proposal and the strategic rationale and timing for a proposed transaction, including the consolidation in the CRO industry. No specific revisions to the ICON January 23, 2021 proposal were discussed, but Mr. Shannon noted that any revised proposal would need to both address the fundamental value of PRA and be sufficient to induce the PRA board of directors to move forward on a pre-emptive basis. Mr. Shannon also noted that the PRA board of directors believed that more than one seat on the combined company board of directors should be filled by a current PRA director. Dr. Cutler said he would discuss with the ICON board of directors, and that Mr. Shannon continuing as a director on the combined company board of directors was a prerequisite for ICON pursuing the proposed transaction.

Later on January 27, 2021, representatives of each of BofA Securities and UBS held a telephone conversation with representatives of Centerview. During the conversation, representatives of Centerview verbally communicated

a revised proposal of $162.50 per share of PRA common stock, comprised of $80 in cash and a number of ICON ordinary shares representing a value of $82.50 based on a fixed exchange ratio determined by the 20-trading day VWAP of ICON ordinary shares ending the day prior to signing. The Centerview representatives reiterated that ICON envisioned Mr. Shannon joining the combined company board of directors. This proposal is referred to as the ICON January 27, 2021 proposal. The Centerview representatives further indicated that the ICON January 27, 2021 proposal and ICON’s willingness to proceed was conditioned on PRA agreeing to the proposed exclusivity agreement.

In the afternoon of January 27, 2021, the PRA board of directors held a special meeting. Representatives of PRA’s outside legal counsel were present. The PRA board of directors discussed the ICON January 27, 2021 proposal, the fact that ICON had consistently reiterated that any proposal was conditioned on exclusivity, their belief that ICON would not be willing to offer more than $165 per share, the low likelihood that other strategic or private buyers would be able to pay a greater price than ICON was currently offering and that the PRA board of directors believed ICON to be a strong counterparty and placed a high value on ICON ordinary shares and the potential upside that PRA stockholders could realize through their ability to retain ownership in the combined company. A representative from PRA’s outside legal counsel then reviewed with the PRA board of directors the proposed terms of the exclusivity agreement and its fiduciary duties under applicable law with respect to entering into such agreement. The PRA board of directors discussed the strategic rationale of the proposed transaction and the financial analyses and considerations with respect to the potential transaction that representatives of each of BofA Securities and UBS had presented at various meetings, including the CRO industry and PRA’s and ICON’s relative positioning in the industry, that the combined company would be able to offer clients more robust offerings across more geographic areas, the advantages of a combination with ICON as opposed to alternative transactions and the financial prospects of PRA in light of the long range projections. The PRA board of directors also discussed the drawbacks of entering into the exclusivity agreement, including that during the exclusivity period PRA would not be able to conduct a market check or enter into alternative transactions with other potential counterparties. In light of the foregoing, the PRA board of directors determined that it was in the best interests of PRA and its stockholders to enter into the proposed exclusivity agreement in order to continue discussions and negotiations with ICON regarding a potential transaction and to authorize BofA Securities and UBS to inform Centerview that the PRA board of directors would be willing to move forward towards a transaction with ICON, and to grant exclusivity through February 24, 2021, at an offer price per share of $165, comprised of $80 in cash and the remainder in ICON ordinary shares.

That evening, as directed by the PRA board of directors, representatives of each of BofA Securities and UBS held a telephone conversation with representatives of Centerview during which they provided feedback on the ICON January 27, 2021 proposal consistent with the PRA board of directors’ authorization.

On January 28, 2021, representatives of each of BofA Securities and UBS held a telephone conversation with representatives of Centerview. During the conversation, representatives of Centerview verbally communicated a revised proposal of $165 per share of PRA common stock, comprised of $80 in cash and a number of ICON ordinary shares representing a value of $85.00 based on a fixed exchange ratio determined by the 20-trading day VWAP of ICON ordinary shares ending the day prior to signing. This proposal is referred to as the ICON January 28, 2021 proposal. The Centerview representatives reiterated that ICON envisioned Mr. Shannon joining the combined company board of directors, and that the ICON January 28, 2021 proposal and ICON’s willingness to proceed was conditioned on PRA agreeing to the proposed exclusivity agreement.

Later on January 28, 2021, Mr. Shannon and Dr. Cutler had a telephone conversation to discuss the ICON January 28, 2021 proposal. During the conversation, Mr. Shannon confirmed that the PRA board of directors was willing to move forward towards a transaction with ICON on the basis of the January 28, 2021 proposal.

On January 29, 2021, a representative of PRA’s outside legal counsel provided comments to ICON’s outside legal counsel with respect to the draft exclusivity agreement.

Also on January 29, 2021, representatives of PRA’s outside legal counsel and members of PRA’s Compensation Committee discussed the treatment of equity awards and other retention and severance matters in connection with the potential transaction.

On January 30, 2021, a representative of ICON’s outside legal counsel sent a revised draft of the exclusivity agreement to PRA’s outside legal counsel and, the following morning, on February 1, 2021, PRA and ICON entered into the exclusivity agreement. The exclusivity agreement provided for exclusive negotiations between PRA and ICON with respect to a strategic transaction until February 24, 2021.

During the period beginning January 30, 2021 through the date of the execution of the merger agreement, representatives of each of PRA and ICON and their respective advisors conducted extensive due diligence with respect to each other’s businesses, financial, commercial, technology, legal and other matters and held discussions concerning their respective businesses, prospects, key standalone and pro forma opportunities and risks and potential synergies that could result from a potential combination. As part of the due diligence process, PRA and ICON made available to each other and their respective advisors an electronic data room containing certain business, financial, commercial, technology, legal and other information of such party. To facilitate each party’s due diligence review, on February 2, 2021, PRA and ICON entered into a Clean Team Confidentiality Agreement as a supplement to the mutual confidentiality agreement entered into on February 28, 2020, which allowed the advisors of each of PRA and ICON and certain PRA and ICON employees to obtain access to additional information. Additionally, during this same period, ICON, PRA and their respective advisors engaged numerous mutual due diligence video and telephonic conferences.

On February 4, 2021, at a regularly scheduled meeting of the PRA board of directors, members of PRA management updated the PRA board of directors on discussions with ICON, accounting due diligence of ICON as well as on proposed key terms of the merger agreement and the ongoing antitrust analysis of the proposed transaction. Following such discussion, the PRA board of directors authorized PRA’s senior management team to continue discussions and negotiations with ICON in accordance with the parameters discussed.

On February 5, 2021, the PRA Compensation Committee held a special meeting. Representatives of PRA’s outside legal counsel were present. The PRA Compensation Committee discussed the treatment of equity awards and other retention and severance matters in connection with the potential transaction. The PRA Compensation Committee authorized PRA’s outside legal counsel to discuss and negotiate such matters with ICON in accordance with the parameters discussed.

On February 7, 2021, a representative of PRA’s outside legal counsel delivered to ICON’s outside legal counsel the PRA Compensation Committee’s desired treatment of PRA’s equity compensation awards and other retention and severance matters in connection with the potential transaction.

On February 8, 2021, members of the senior management teams of each of ICON and PRA had a due diligence meeting by video conference. At the meeting, members of PRA management presented an overview of PRA’s business and members of ICON management asked questions of PRA management. Members of the senior management teams of ICON and PRA also discussed, among other things, potential achievable revenue synergies from combining their businesses.

On February 10, 2021, members of the senior management teams of each of ICON and PRA had a due diligence meeting by video conference. At the meeting, members of ICON management presented an overview of ICON’s business and members of PRA management asked questions of ICON management. Representatives of the senior management teams of ICON and PRA also discussed, among other things, potential revenue synergies that might be realized by combining their businesses.

That evening, a representative of ICON’s outside legal counsel delivered to PRA’s outside legal counsel an initial draft merger agreement. Among other things, the draft included provisions (i) providing for Mr. Shannon to join the combined company board of directors, (ii) requiring each of PRA and ICON not to solicit alternative transactions, except that the PRA and ICON boards of directors could change their respective recommendations to support the merger or the share issuance, as applicable, to its stockholders if they determine a proposal from a third party to be a superior proposal, (iii) capping the amount of divestitures that either ICON or PRA would be required to make in order to obtain regulatory approvals at assets representing $50 million of revenue in 2020 and (iv) granting ICON the right to terminate the merger agreement in the event of certain adverse tax law changes or to enter into an alternative transaction (in each case, without paying a termination fee). The draft merger agreement included proposed termination fees of 4% of the transaction value for both ICON and PRA. From this point until the merger agreement was finalized, the parties negotiated the terms of the merger agreement, including with respect to the points described in this paragraph, consistent with the authorizations and parameters provided by the PRA and ICON boards of directors. For additional information regarding the final terms of the merger agreement, see the section entitled “The Merger Agreement” beginning on page 116 and the copy of the merger agreement attached as Annex A to this joint proxy statement/prospectus.

From February 11, 2021 through February 18, 2021, members of the senior management teams of each of ICON and PRA held further due diligence meetings by teleconference and video conference, covering areas of the respective businesses as well as synergies analysis with respect to the proposed combination.

On February 12, 2021, representatives of each of ICON’s outside legal counsel and PRA’s outside legal counsel discussed the terms of the merger agreement and the areas of focus for their respective clients.

On February 17, 2021, a representative of PRA’s outside legal counsel delivered to ICON’s outside legal counsel a revised draft of the merger agreement. Among other revisions, the revised draft merger agreement (i) removed ICON’s ability to change its recommendation or terminate the merger agreement in the event of a superior proposal while granting PRA such a termination right, (ii) provided that the boards of directors of both ICON and PRA have the ability to change their respective recommendations upon the occurrence of certain intervening events, (iii) increased the cap on required divestitures to obtain antitrust approval to those that would have a material adverse effect on the combined company, (iv) removed ICON’s ability to terminate the merger agreement in the event of an adverse tax law change or to enter into an alternative transaction, (v) made revisions to the termination fee triggers and amounts and (vi) left open the contemplated number of seats on the combined company board of directors to be held by current PRA directors and who would hold such seat(s).

On February 18, 2021, Mr. Shannon and Dr. Cutler had a telephone conversation during which they discussed next steps related to the proposed transaction and agreed to instruct PRA and ICON’s respective internal legal teams and outside advisors to continue mutual diligence and to finalize and negotiate definitive agreements expeditiously. They tentatively agreed to work towards an announcement date of February 24, 2021. Also on February 18, 2021, representatives of ICON’s outside legal counsel outlined for PRA’s outside legal counsel the key issues and areas of focus raised by the February 17 draft of the merger agreement from their perspective. Following that discussion, representatives of PRA’s outside legal counsel had a call with members of PRA senior management during which they discussed the issues raised by ICON’s outside legal counsel. Thereafter, representatives of PRA’s outside legal counsel conveyed PRA’s initial feedback on such items to ICON’s outside legal counsel.

On February 19, 2021, the PRA board of directors held a special meeting. Members of PRA’s senior management team and representatives of PRA’s outside legal counsel also attended the meeting. A member of the PRA senior management team discussed quality of earnings due diligence on ICON performed by PRA’s outside accountants as well as ICON’s projections, the impact of COVID-19 on ICON’s business and analyst consensus on ICON’s prospects. Mr. Shannon and the member of PRA management then presented an overview of the CRO landscape, a breakdown of the strategic rationale and a review of precedent transactions. A representative from PRA’s outside legal counsel provided an update on the ongoing antitrust analysis of the proposed transaction. Another representative of PRA’s outside legal counsel then described updates to the merger agreement, including proposed responses to ICON’s initial feedback, and outlined fiduciary duty considerations relating to the PRA board of directors’ evaluation of the proposed transaction. The PRA board of directors concurred with the proposals presented by the representatives of its outside legal counsel. During an executive session, the PRA board of directors discussed the Compensation Committee’s desired treatment of equity compensation awards and other retention and severance matters in connection with the proposed transaction. Later on February 19, 2021, representatives of each of ICON’s outside legal counsel and PRA’s outside legal counsel discussed the terms of the merger agreement and representatives of PRA’s outside legal counsel conveyed the PRA board of directors’ views on the issues raised the previous day.

On February 19, 20 and 21, 2021, representatives of PRA’s outside legal counsel and members of PRA’s Compensation Committee discussed the status of negotiations with representatives of ICON’s outside legal counsel regarding treatment of equity awards and other retention and severance matters in connection with the potential transaction. On those days, representatives of PRA’s outside legal counsel had a number of discussions with representatives of ICON’s outside legal counsel regarding the PRA Compensation Committee’s desired treatment of PRA’s equity compensation awards and other retention and severance matters in connection with the potential transaction.

On February 22, 2021, a representative of ICON’s outside legal counsel delivered to PRA’s outside legal counsel a revised draft of the merger agreement. Among other revisions, the revised draft merger agreement reinserted provisions providing for Mr. Shannon to join the combined company board of directors as the sole additional member and revisions to the termination fee triggers and amounts. Later that day, following discussions with members of PRA

management and representatives of ICON’s outside legal counsel, a representative of PRA’s outside legal counsel delivered to ICON’s outside legal counsel a revised draft of the merger agreement. Among other revisions, the revised merger agreement provided for Mr. Shannon as well as one other PRA representative to join the combined company board of directors.

That same day, the PRA board of directors held a special meeting. Members of PRA’s senior management team and representatives of each of BofA Securities, UBS and PRA’s outside legal counsel also attended the meeting. A representative of PRA’s outside legal counsel outlined fiduciary duty considerations relating to the PRA board of directors’ evaluation of the proposed transaction and summarized the changes made by ICON to the merger agreement. Representatives of each of BofA Securities and UBS reviewed certain financial aspects of the proposed transaction, including that ICON proposed to determine the fixed exchange ratio using a 19-trading day VWAP of ICON ordinary shares ending two trading days prior to signing. Representatives of each of BofA Securities and UBS then reviewed with the PRA board of directors their respective preliminary financial analyses of the proposed merger consideration based on PRA’s and ICON’s respective financial forecasts, synergy estimates and current market data, all of which had been previously reviewed and approved by the PRA board of directors for use by each of BofA and UBS Securities in connection with their respective financial analyses.

The PRA board of directors then discussed ICON’s proposal in light of the new draft of the merger agreement and the financial analyses of the financial advisors and provided parameters within which they authorized PRA’s outside legal counsel to negotiate the remaining open terms of the merger agreement.

From that meeting until the early morning of February 24, 2021, representatives of each of ICON’s outside legal counsel and PRA’s outside legal counsel discussed the open terms of the merger agreement and resolved the remaining open legal points, as authorized by their respective clients.

Before the opening of trading on February 24, 2021, the PRA board of directors held a special meeting. Members of PRA’s senior management team and representatives of each of BofA Securities, UBS and PRA’s outside legal counsel attended the meeting. A representative of PRA’s outside legal counsel summarized the final terms of the merger agreement. Representatives of BofA Securities presented to the PRA board of directors BofA Securities financial analyses of the merger consideration. Following the conclusion of this presentation and upon the request of the PRA board of directors, BofA Securities rendered its oral opinion, which was subsequently confirmed by delivery of a written opinion dated February 24, 2021, to the effect that, as of the date of the opinion and based on and subject to various assumptions and limitations described in BofA Securities’ written opinion, the merger consideration to be received in the merger by PRA stockholders was fair, from a financial point of view, to such holders, as more fully described below under the section entitled “—Opinions of PRA’s Financial Advisors—BofA Securities, Inc.” beginning on page 16. Representatives of UBS then reviewed orally with the PRA board of directors UBS’ financial analyses of the merger consideration. Following the conclusion of this presentation and upon the request of the PRA board of directors, UBS rendered its oral opinion, subsequently confirmed by delivery of a written opinion dated February 24, 2021, to the effect that, as of that date and based upon and subject to various assumptions, matters considered and limitations described in its written opinion, the merger consideration to be received by PRA stockholders in the merger was fair, from a financial point of view, to such holders, as described in the section entitled “—Opinions of PRA’s Financial Advisors—UBS Securities LLC” beginning on page 17. Following discussion, and after carefully considering the proposed terms of the transaction, and taking into consideration the matters discussed during that meeting and at prior meetings and the factors described below under “Recommendation of the PRA Board of Directors and Reasons for the Merger”, on a motion duly made and seconded, the PRA board of directors unanimously: (i) declared the merger agreement and the transactions contemplated thereby, including the merger, fair to, and in the best interests of PRA and the PRA stockholders, (ii) approved and declared advisable the merger agreement and the transactions contemplated thereby, including the merger, (iii) directed that the merger agreement be submitted to the PRA stockholders for adoption at a meeting of such stockholders and (iv) recommended that the PRA stockholders vote in favor of the adoption of the merger agreement.

On the morning of February 24, 2021, PRA and ICON executed the merger agreement, the execution of which was announced in a press release issued by ICON soon thereafter.

By unanimous vote, the PRA board of directors, at a meeting held on February 24, 2021, (a) determined that the merger agreement and the transactions contemplated thereby, including the merger, are fair to, and in the best interests of, PRA and the PRA stockholders, (b) approved and declared advisable the merger agreement and the

transactions contemplated thereby, including the merger, and (c) resolved to recommend that the PRA stockholders approve and adopt the merger agreement and the transactions contemplated thereby, including the merger. The PRA board of directors unanimously recommends that PRA stockholders vote “FOR” the merger proposal and “FOR” the non-binding compensation advisory proposal.

In the course of reaching its determination and recommendation, the PRA board of directors met several times to consider a potential transaction with ICON, including in executive session, and consulted with PRA’s senior management, outside legal counsel and financial advisors. In recommending that PRA stockholders vote their shares of PRA common stock in favor of adoption of the merger agreement, the PRA board of directors also considered a number of factors, including the following factors (not necessarily in order of relative importance) which the PRA board of directors viewed as being generally positive or favorable in coming to its determination and recommendation:

The PRA board of directors also considered a number of uncertainties, risks and factors it deemed generally negative or unfavorable in making its determination, approval and related recommendation, including the following (not necessarily in order of relative importance):

The PRA board of directors believed that, overall, the potential benefits of the merger to PRA stockholders outweighed the risks and uncertainties of the merger.

The foregoing discussion of factors considered by the PRA board of directors in reaching its conclusions and recommendation includes the principal factors considered by the PRA board of directors, but is not intended to be exhaustive and may not include all of the factors considered by the PRA board of directors, but includes the material factors considered by the PRA board of directors. In light of the variety of factors considered in connection with its evaluation of the merger, the PRA board of directors did not find it practicable to, and did not, quantify or otherwise assign relative or specific weights to the specific factors considered in reaching its determinations and

recommendations. Rather, the PRA board of directors based its decisions on the totality of the factors and information it considered. Moreover, each member of the PRA board of directors applied his or her own personal business judgment to the process and may have given different weight to different factors.

At a special meeting held on February 23, 2021, the ICON board of directors unanimously approved and declared advisable the merger agreement and the transactions contemplated by the merger agreement, including the merger and the ICON share issuance, on the terms and subject to the conditions set forth in the merger agreement. In the course of reaching its determination, the ICON board of directors considered a number of factors, including the following:

The foregoing statements are forward-looking in nature and should be read in conjunction with the discussion entitled “Cautionary Note Regarding Forward-Looking Statements” beginning on page 32 and the section entitled “Risk Factors” beginning on page 34. There can be no assurance that any anticipated strategic opportunities will be realized.

ICON shareholders are not required to approve the adoption of the merger agreement under Irish law and, accordingly, ICON shareholders are not being asked to vote on the merger or the adoption of the merger agreement.

On February 23, 2021, Centerview rendered to the ICON board of directors its oral opinion, subsequently confirmed in a written opinion dated February 23, 2021, that, as of such date and based upon and subject to various assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion, the Consideration to be paid by ICON and its subsidiaries pursuant to the merger agreement was fair, from a financial point of view, to ICON.

The full text of Centerview’s written opinion, dated February 23, 2021, which describes the assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion, is attached as Annex B and is incorporated herein by reference, and the summary of the written opinion of Centerview set forth below is qualified in its entirety by reference to such full text. Centerview’s financial advisory services and opinion were provided for the information and assistance of the ICON board of directors (in their capacity as directors and not in any other capacity) in connection with and for purposes of its consideration of the Transaction and Centerview’s opinion only addressed the fairness, from a financial point of view, as of the date thereof, to ICON of the Consideration to be paid by ICON and its subsidiaries pursuant to the

merger agreement. Centerview’s opinion did not address any other term or aspect of the merger agreement or the Transaction and does not constitute a recommendation to any ICON shareholder or any other person as to how such shareholder or other person should vote with respect to the merger or otherwise act with respect to the Transaction or any other matter. The full text of Centerview’s written opinion should be read carefully in its entirety for a description of the assumptions made, procedures followed, matters considered, and qualifications and limitations upon the review undertaken by Centerview in preparing its opinion.

In connection with rendering the opinion described above and performing its related financial analyses, Centerview reviewed, among other things:

Centerview also participated in discussions with members of the senior management and representatives of ICON and PRA regarding their assessment of the PRA Internal Data, the ICON-Adjusted PRA Forecasts, ICON Internal Data (including, without limitation, the ICON Forecasts), the Synergies and the strategic rationale for the Transaction. In addition, Centerview reviewed publicly available financial and stock market data, including valuation multiples, for PRA and ICON and compared that data with similar data for certain other companies, the securities of which are publicly traded, in lines of business that Centerview deemed relevant. Centerview also compared certain of the proposed financial terms of the Transaction with the financial terms, to the extent publicly available, of certain other transactions that Centerview deemed relevant, and conducted such other financial studies and analyses and took into account such other information as Centerview deemed appropriate.

Centerview assumed, without independent verification or any responsibility therefor, the accuracy and completeness of the financial, legal, regulatory, tax, accounting and other information supplied to, discussed with, or reviewed by Centerview for purposes of its opinion and, with ICON’s consent, Centerview relied upon such

information as being complete and accurate. In that regard, Centerview assumed, at ICON’s direction, that the PRA Internal Data, the ICON-Adjusted PRA Forecasts, the ICON Internal Data (including, without limitation, the ICON Forecasts) and the Synergies were reasonably prepared on bases reflecting the best currently available estimates and judgments of the management of ICON and PRA, as applicable, as to the matters covered thereby and Centerview relied, at ICON’s direction, on the PRA Internal Data, the ICON-Adjusted PRA Forecasts, the ICON Internal Data (including, without limitation, the ICON Forecasts) for purposes of Centerview’s analysis and opinion. Centerview expressed no view or opinion as to the PRA Internal Data, the ICON-Adjusted PRA Forecasts, the ICON Internal Data (including, without limitation, the ICON Forecasts) or the Synergies or the assumptions on which it was based. In addition, at ICON’s direction, Centerview did not make any independent evaluation or appraisal of any of the assets or liabilities (contingent, derivative, off-balance-sheet or otherwise) of PRA or ICON, nor was Centerview furnished with any such evaluation or appraisal, and was not asked to conduct, and did not conduct, a physical inspection of the properties or assets of PRA or ICON. Centerview assumed, at ICON’s direction, that the final executed merger agreement would not differ in any respect material to Centerview’s analysis or opinion from the Draft Agreement reviewed by Centerview and that the representations and warranties made by each of PRA, ICON, US HoldCo and Merger Sub in the Draft Agreement are and will be true and correct in all respects material to Centerview’s analysis. Centerview also assumed, at ICON’s direction, that the Transaction will be consummated on the terms set forth in the Draft Agreement and in accordance with all applicable laws and other relevant documents or requirements, without delay or the waiver, modification or amendment of any term, condition or agreement, the effect of which would be material to Centerview’s analysis or Centerview’s opinion and that, in the course of obtaining the necessary governmental, regulatory and other approvals, consents, releases and waivers for the Transaction, no delay, limitation, restriction, condition or other change will be imposed, the effect of which would be material to Centerview’s analysis or Centerview’s opinion. Centerview also assumed that the Transaction will have the tax consequences described in discussions with, and materials furnished to Centerview by, representatives of ICON. Centerview did not evaluate and did not express any opinion as to the solvency or fair value of PRA or ICON, or the ability of PRA or ICON to pay its obligations when they come due, or as to the impact of the Transaction on such matters, under any state, federal or other laws relating to bankruptcy, insolvency or similar matters. Centerview is not a legal, regulatory, tax or accounting advisor, and Centerview expressed no opinion as to any legal, regulatory, tax or accounting matters.

Centerview’s opinion expressed no view as to, and did not address, ICON’s underlying business decision to proceed with or effect the Transaction, or the relative merits of the Transaction as compared to any alternative business strategies or transactions that might be available to ICON or in which ICON might engage. Centerview’s opinion was limited to and addressed only the fairness, from a financial point of view, as of the date of Centerview’s written opinion, to ICON of the Consideration to be paid by ICON and its subsidiaries pursuant to the merger agreement. For purposes of its opinion, Centerview was not asked to, and Centerview did not, express any view on, and its opinion did not address, any other term or aspect of the merger agreement or the Transaction, including, without limitation, the structure or form of the Transaction, or any other agreements or arrangements contemplated by the merger agreement or entered into in connection with or otherwise contemplated by the Transaction, including, without limitation, the fairness of the Transaction or any other term or aspect of the Transaction to, or any consideration to be received in connection therewith by, or the impact of the Transaction on, the holders of any class of securities, creditors or other constituencies of ICON or any other party. In addition, Centerview expressed no view or opinion as to the fairness (financial or otherwise) of the amount, nature or any other aspect of any compensation to be paid or payable to any of the officers, directors or employees of PRA or any party, or class of such persons in connection with the Transaction, whether relative to the Consideration to be paid by ICON and its subsidiaries pursuant to the merger agreement or otherwise. Centerview’s opinion related, in part, to the relative values of PRA and ICON. Centerview’s opinion was necessarily based on financial, economic, monetary, currency, market and other conditions and circumstances as in effect on, and the information made available to Centerview as of, the date of Centerview’s written opinion, and Centerview does not have any obligation or responsibility to update, revise or reaffirm its opinion based on circumstances, developments or events occurring after the date of Centerview’s written opinion. Centerview’s opinion expressed no view or opinion as to what the value of ICON ordinary shares actually will be when issued pursuant to the Transaction or the prices at which shares of PRA common stock or ICON ordinary shares will trade or otherwise be transferable at any time, including following the announcement or consummation of the Transaction. Centerview’s opinion does not constitute a recommendation to any shareholder of ICON or any other person as to how such shareholder or other person should vote with respect to the merger or otherwise act with respect to the Transaction or any other matter.

Centerview’s financial advisory services and its written opinion were provided for the information and assistance of the ICON board of directors (in their capacity as directors and not in any other capacity) in connection with and for purposes of its consideration of the Transaction. The issuance of Centerview’s opinion was approved by the Centerview Partners LLC Fairness Opinion Committee.

The following is a summary of the material financial analyses prepared and reviewed with the ICON board of directors in connection with Centerview’s opinion, dated February 23, 2021. The summary set forth below does not purport to be a complete description of the financial analyses performed or factors considered by, and underlying the opinion of, Centerview, nor does the order of the financial analyses described represent the relative importance or weight given to those financial analyses by Centerview. Centerview may have deemed various assumptions more or less probable than other assumptions, so the reference ranges resulting from any particular portion of the analyses summarized below should not be taken to be Centerview’s view of the actual value of ICON or PRA. Some of the summaries of the financial analyses set forth below include information presented in tabular format. In order to fully understand the financial analyses, the tables must be read together with the text of each summary, as the tables alone do not constitute a complete description of the financial analyses performed by Centerview. Considering the data in the tables below without considering all financial analyses or factors or the full narrative description of such analyses or factors, including the methodologies and assumptions underlying such analyses or factors, could create a misleading or incomplete view of the processes underlying Centerview’s financial analyses and its opinion. In performing its analyses, Centerview made numerous assumptions with respect to industry performance, general business and economic conditions and other matters, many of which are beyond the control of ICON, PRA or any other parties to the Transaction. None of ICON, PRA, US HoldCo, Merger Sub or Centerview or any other person assumes responsibility if future results are materially different from those discussed. Any estimates contained in these analyses are not necessarily indicative of actual values or predictive of future results or values, which may be significantly more or less favorable than as set forth below. In addition, analyses relating to the value of ICON or PRA do not purport to be appraisals or reflect the prices at which ICON or PRA may actually be sold. Accordingly, the assumptions and estimates used in, and the results derived from, the financial analyses are inherently subject to substantial uncertainty. Except as otherwise noted, the following quantitative information, to the extent that it is based on market data, is based on market data as it existed on or before February 23, 2021 and is not necessarily indicative of current market conditions. For purposes of the financial analyses described below in this section, the terms Adjusted EBITDA and Adjusted EPS are defined in the section entitled “ICON Unaudited Financial Forecasts” beginning on pages 107.

Centerview reviewed and compared certain financial information, ratios and multiples for PRA to corresponding financial information, ratios and multiples for selected publicly traded companies that Centerview deemed comparable, based on its experience and professional judgment, to PRA, which we refer to in this “Opinion of ICON’s Financial Advisor” section as the selected trading companies for PRA.

The selected trading companies for PRA consisted of:

Although none of the selected trading companies for PRA is directly comparable to PRA, these companies were selected by Centerview, among other reasons, because they are publicly traded companies in the contract research organization industry with certain operational and financial characteristics that Centerview, in its experience and professional judgment, deemed generally relevant for comparative purposes.

Using publicly available information obtained from SEC filings, publicly available Wall Street research, other publicly available information and closing stock prices as of February 22, 2021, as well as the ICON Forecasts and

the ICON-Adjusted PRA Forecasts, Centerview calculated, for PRA and for each of its selected trading companies, the following ratios and multiples: (1) the enterprise value (calculated as the market value of fully diluted common equity with outstanding options treated with the treasury stock method plus the face value of debt and non-controlling interests, less cash and cash equivalents including investments in debt, equity and other securities) as a multiple of 2021 calendar year estimated Adjusted EBITDA and (2) the price to earnings ratio for the 2021 calendar years based on consensus analyst estimates (adjusted to exclude expenses related to stock-based compensation).

The results of this analysis for the selected trading companies for PRA are summarized below:

Based on this analysis and other considerations that Centerview deemed relevant in its experience and professional judgment, for purposes of its analysis Centerview selected a reference range of estimated 2021 price to earnings ratios (excluding stock-based compensation expense) of 18.0x to 26.0x and a reference range of enterprise value to estimated 2021 Adjusted EBITDA multiples of 14.0x to 19.0x. However, because none of the selected companies is exactly the same as PRA, Centerview believed that it was inappropriate to, and therefore did not, rely solely on the quantitative results of the selected company analysis. Accordingly, in selecting these reference ranges, Centerview made qualitative judgments based on its experience and professional judgment concerning differences between the business, financial and operating characteristics and prospects of PRA and the selected trading companies for PRA that could affect the public trading values in order to provide a context in which to consider the results of the quantitative analysis.

Centerview applied the Adjusted EBITDA multiple reference ranges to PRA’s calendar year 2021 estimated Adjusted EBITDA as set forth in the ICON-Adjusted PRA Forecasts to derive ranges of implied enterprise values for PRA. Centerview also applied the Adjusted EPS multiple reference ranges to PRA’s calendar year 2021 estimated Adjusted EPS as set forth in the ICON-Adjusted PRA Forecasts to derive ranges of implied values per share of PRA common stock. Centerview subtracted from each of these ranges of implied enterprise values the value of PRA’s net debt (including non-controlling interest) as of December 31, 2020 as set forth in the PRA Internal Data to derive a range of implied equity values for PRA. Centerview then divided these implied equity values by the number of fully-diluted outstanding shares of PRA common stock as of February 19, 2021 as set forth in the PRA Internal Data to derive a range of implied values per share of PRA common stock.

The results of this analysis, rounded to the nearest $1.00, are summarized below:

Centerview then compared these ranges to the value of the Consideration of $165.90 per share of PRA common stock, based on the closing price of ICON ordinary shares as of February 22, 2021, and implied by the sum of (i) 0.4125 of one share of ICON ordinary shares and (ii) $80.00 cash consideration per share of PRA common stock to be paid in the merger.

Centerview analyzed certain information relating to selected precedent transactions in the contract research organization industry in the past fourteen (14) years that Centerview deemed, based on its experience and professional judgment, to be comparable to PRA and the Transaction.

The selected precedent transactions consisted of: