He played for Barnhall, Bective Rangers, St Marys College RFC, Leinster, Stade Toulousain and Ireland, being capped 13 times.

Brennan made his debut for the senior Ireland team in a 13-37 defeat against South Africa on 13 June 1998[1] and went on to make 13 test appearances, including the 1999 Rugby World Cup.

His uncle took this further alleging sectarian abuse had taken place, a claim initially supported by the club before swiftly being retracted.

Brennan has had occasional columns, written by Gerry Thornley, published in the Irish Times, and has appeared on television as a rugby commentator.

In 2007, Brennan's autobiography Heart and Soul, ghost-written by Gerry Thornley, won the William Hill Irish Sports Book of the Year.