[3] In 1979, Brennan supported George Colley in the Fianna Fáil leadership election caused by the retirement of Jack Lynch.

[4] In the early 1980s, he was a prominent member of the Gang of 22 who tried unsuccessfully to wrest control of the Fianna Fáil party from Haughey.

It was widely expected that Brennan would join the Progressive Democrats when they were founded by O'Malley in 1985, but he remained within Fianna Fáil.

In 1987, Fianna Fáil returned to office and Brennan was appointed Minister of State at the Department of Industry and Commerce with responsibility for trade and marketing.

In January 1993, a Fianna Fáil–Labour Party coalition came to office and Brennan was demoted to Minister of State at the Department of Enterprise and Employment with responsibility for commerce and technology.

[2] After the 2007 general election, he played a key role in the negotiations with the Green Party which led to the formation of the new Government.

He supported legal reforms such as the legalisation of divorce and the unrestricted availability of contraception, advocating for greater social freedoms.

On economic matters, Brennan championed market-driven policies aimed at encouraging enterprise and reducing state intervention in the economy.

Brennan also sought to modernise Ireland's infrastructure, playing a key role in transport projects and pushing for reforms in the education and welfare systems.

Taoiseach Brian Cowen said Brennan would be remembered as "a brilliant political strategist, a dedicated constituency TD, a reforming minister and a very popular colleague".