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Mark O'Sullivan is praised for his "impressive technical ability".

Who’s Who Legal 100

Expertise

Mark O’Sullivan is a partner in the firm’s Tax Department and advises on all aspects of Irish corporate taxation.  His primary focus is advising overseas clients establishing operations and doing business in and through Ireland.  Mark also advises extensively on all aspects of international tax planning, including IP planning, cross-border reorganizations and financing transactions.

Mark was previously seconded to the legal department of a large investment bank in London in 2004.  Mark was based in the firm’s Palo Alto office from 2007-2020 and now splits his time between Dublin and Palo Alto.

Mark is a vice-chair of the Foreign Lawyer’s Forum of the Tax Section of the American Bar Association (ABA), and is also actively involved in the International Fiscal Association (IFA). Mark regularly speaks on international tax issues and has also published articles in Tax Notes International, the International Tax Review, the Irish Taxation Review, IFLR, BNA and Finance Magazine.  Mark has also guest lectured on international tax matters at the University of Washington and San Jose State University.

Accolades
Mark O'Sullivan operates out of Palo Alto and San Francisco and is recognised for his deep knowledge of commercial tax matters in the Irish market. One client reports "He is responsive, attentive and thorough".
Tax: Chambers Europe 2019

Mark O'Sullivan is highly regarded for his “expertise in relation to corporate taxation matters in Ireland, regularly advising international clients on inward investment into the country.”
Tax: Chambers Europe 2017 

Mark O'Sullivan is praised for his “assistance on strategy and regulatory issues.”
Tax: Chambers Europe 2017

Interviewees appreciate Mark's "help with strategy and regulatory issues. It has been really superb." 
Tax: Chambers Europe 2016

Mark O'Sullivan is recognised as a Tax Expert
Euromoney Expert Guide 2015

Mark O'Sullivan is widely acclaimed for his strength in advising overseas clients on international tax planning and cross-border reorganisation cases involving Irish tax issues.
Tax: Chambers Europe 2015
Education

Law Society of Ireland

University College Dublin and Université de Toulouse I (France), (Bachelor of Civil Law)

University College Dublin, (Diploma in Financial Services Law)

AITI / Chartered Tax Adviser (CTA)