Club Brennan won his first
Dublin Senior Football Championship medal with St Vincents in 2007 in the final against
St Brigid's at
Parnell Park. The team then went on to win the
Leinster Senior Club Football Championship final against Tyrrellspass of
Westmeath. Brennan was chosen as the Leinster club player of the year for his performances with St Vincents. Brennan won the 2008
All-Ireland Senior Club Football Championship with
St Vincents in a hard-fought game. He won the
Dublin Senior Football Championship with St Vincents, as
captain, in 2013.
Inter-county Brennan made his national league debut for Dublin against
Tyrone on 3 February 2007. He was a member of the Dublin team that won the
2007 O'Byrne Cup, playing against
Laois at
O'Connor Park in Offaly. The game finished on a scoreline of 1–18 to 2–13. He finished the tournament with a total of 0–4. In 2008, he retired from Dublin's football panel, citing burnout and fatigue as his reasons. However, on 30 July 2008, he was named in Dublin's panel for the quarter-final and accepted a return. He was part of the Dublin panel that won the
2011 All-Ireland Senior Football Championship (SFC) title. He won a second All-Ireland SFC title with Dublin in 2013, scoring two points in the
final against
Mayo. In October 2015, Brennan announced his retirement from inter-county football, but he continued to play club football. ==Coaching==