Mark O'Sullivan


Palo Alto and San Francisco

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

Who’s Who Legal 100


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 has been based in the firm’s Palo Alto office since 2007 and is registered as a Foreign Legal Consultant with the California State Bar

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 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.


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 Global 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".
Chambers Global 2017

Clients reserve particular praise for Mark O’Sullivan’s assistance on strategy and regulatory issues.
Chambers Global 2017

Interviewees appreciate Mark O’Sullivan’s "help with strategy and regulatory issues. It has been really superb.
Chambers Europe 2016

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.
Chambers Global & Europe 2015

Mark O’Sullivan specialises in all aspects of Irish corporate taxation and received praise for his "impressive technical ability".
Who’s Who Legal 100 2014: Corporate Tax expert

Based in the firm's Palo Alto office, Mark O'Sullivan is notably experienced in advising US companies on Irish tax issues. He recently advised Eaton on the tax aspects of its acquisition of Cooper Industries.
Chambers Global 2014

The well-regarded Mark O'Sullivan operates from the firm's Palo Alto office and advises US companies on foreign direct investment projects into Ireland.
Chambers Global 2013


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)

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