Frank Flannery has quit his roles within Rehab and Fine Gael.
The resignation of the former Rehab Group chief executive comes amid complaints about his failure to appear before a hearing of the Public Accounts Committee.
In a statement this evening Mr Flannery said his involvement with Rehab was making it the subject of political controversy which he wanted to avoid.
He has also resigned as Fine Gael's director of elections and as a trustee for the party, however he said he will continue to support the party as a member and private citizen.
Mr Flannery has been in the spotlight since failing to appear at a meeting of the Public Accounts Committee two weeks ago, despite being invited as the chairman of Rehab's remuneration committee.
The chairman of the Public Accounts Committee, John Mc Guinness, said Flannery is still expected to attend and take questions from TDs.
McGuinness said he has written to Rehab today urging Mr Flannery to attend a hearing, to discuss his role in setting the salary of CEO Angela Kerins and other senior managers.
I have informed the Chairman of the Rehab Group board, Mr. Brian Kerr, that I wish to step down as a director of the Rehab Group and any other group boards with immediate effect.
It is a step that I undertake with real regret but I have come to the opinion that my involvement with the board is making the Rehab Group the subject of political controversy as this time.
I spent 34 years in the Rehab Group and retired in December 2006. I rejoined the board in 2011.
The Rehab Group has played an enormously valuable role in Irish Society for nearly 70 years and I wish it continued success for the future.
I have also informed the General Secretary of Fine Gael, Mr. Tom Curran, that I am stepping down as Director of Elections and as a Trustee of the party as of today.
My involvement with Fine Gael related only to electoral strategy and organisation and I had no role in advising the government.
Fine Gael has been mandated, along with the Labour Party, with the onerous task of turning the economy around after the deepest recession since 1929.
I believe the government, and Fine Gael in particular, is performing well and the economy is on the mend.
The party and the Taoiseach will continue to have my complete support and I will assist the party in any way I can as a private citizen and as a proud ordinary member of Fine Gael.