A native of Kinlough in Leitrim,[citation needed] Brennan started his inter-county career with Leitrim, but transferred to the neighbouring county of Donegal after falling out with the county board.[citation needed] Brennan won back-to-back FBD League titles with Leitrim in 2013 and 2014.[citation needed][clarification needed]
Brennan was involved with club football in Donegal, and won a Donegal Intermediate Football Championship with Réalt na Mara in 2015.[3] He was described as "one of the driving forces" in the club's IFC win.[4]
Brennan first featured for Donegal at senior level under the management of Rory Gallagher. He made his first competitive start for Donegal against Kerry in the opening round of the 2017 National Football League.[5] He scored a point in that game.[6] He made his championship debut against.[who?]
Brennan made a second half substitute appearance in the final of the 2019 Ulster Senior Football Championship, which Donegal also won.[21] He also made substitute appearances against Fermanagh in the quarter-final and against Tyrone in the semi-final.[22][23]
Brennan started Donegal's first five fixtures of the 2020 National Football League against Mayo, Meath, Galway, Dublin and Monaghan, scoring a point against Mayo in the opening game.[24][25][26][27][28] Then the COVID-19 pandemic brought play to a halt. Play resumed behind closed doors on 18 October with a home game against Tyrone; Brennan started that game as well.[29] He did not participate in the concluding game of the league campaign (away to Kerry) as he and other senior players (such as Michael Murphy, Hugh McFadden, Ryan McHugh and Eoghan Bán Gallagher) were rested ahead of the 2020 Ulster Senior Football Championship quarter-final against Tyrone the following Sunday.[30][31] Brennan started that opening victory against Tyrone, scoring one point.[32] He also started the semi-final victory against Armagh and the final against Cavan, in what proved to be the season's concluding game for his team.[33][34]
Brennan started each of Donegal's opening three fixtures of the 2022 National Football League, against Mayo, Kildare and Kerry respectively.[42][43][44] He did not feature against either Tyrone or Monaghan in the fourth and fifth fixtures.[45][46] He made a substitute appearance away to Dublin in the penultimate fixture, but did not play against Armagh at O'Donnell Park in the next game.[47][48] In the 2022 Ulster Senior Football Championship, he made substitute appearances in two of Donegal's three fixtures, the quarter-final against Armagh and the final against Derry (during extra-time).[49][50] He did not feature in the semi-final against Cavan.[51] He also did not feature in the 2022 All-Ireland Senior Football Championship qualifier loss to Armagh.[52]
Personal life
Brennan's father is Brendan and his mother is Rita.[53] In late-2023, he married Bundoran native and palliative nurse Colleen Granaghan, whose father Seamie played at half-forward during Donegal's 1972 and 1974 Ulster SFC title victories.[53] At that time the couple had two sons.[53]
He is not related to Jamie, with whom he has lined out for club and county.[54]