Career
A farmer, Brennan was first elected as an independent Territorial Decoration to the 5th Dáil at the June 1927 general election and returned to the 6th Dail at the September 1927 general election. After losing his seat at the 1932 general election, Brennan joined Cumann na nGaedheal and was returned to the 8th Dáil at the 1933 general election. He was re-elected at the 1937 general election to the 9th Dáil and at the 1938 general election to the 10th Dáil.