He began his working life as a freelance journalist with The Irish Press and the Donegal Democrat before establishing himself as an auctioneer, estate agent, and spirit merchant.

In 1959, when Seán Lemass succeeded as Taoiseach, Brennan was appointed as Parliamentary Secretary to the Minister for Finance.

After the 1973 general election, Fine Gael and the Labour Party formed a National Coalition government.

Brennan was elected Ceann Comhairle of Dáil Éireann, a position he held until his death in 1980, aged 67.

[4][5] The by-election for his seat in the Donegal constituency was held on 6 November 1980, and won by the Fianna Fáil candidate Clement Coughlan.