[2][3] Brennan played competitive hurling in his final year at St Kieran's College, having earlier come to prominence as a dual player at minor levels with the Graigue–Ballycallan club.
Brennan joined the Graigue-Ballycallan club at a young age and played in all grades at juvenile and underage levels.
[8] On 15 October 2000, Brennan scored two points from play when Graigue–Ballycallan won a second Kilkenny Championship in three years after a 0-16 to 0-09 defeat of O'Loughlin Gaels in the final.
[9] Later that season he won a Leinster Championship medal at left corner-forward after a 0-14 to 1-08 defeat of University College Dublin in the decider.
Brennan made his senior debut for Kilkenny on 26 March 2000, scoring 2-03 in a 2-14 to 2-09 National Hurling League defeat of Tipperary.
[13] Later that season he made his first appearance in the Leinster Championship against Wexford before coming on as a substitute for Henry Shefflin in the final in a 2-21 to 1-13 defeat of Offaly.
[16] On 8 July 2001, Brennan won a second consecutive Leinster Championship medal after being introduced as a second-half substitute for Denis Byrne in 2-19 to 0-12 defeat of Wexford in the final.
[19] On 8 September 2002, Brennan started in his first All-Ireland final and scored one point on the way to collecting his second winners' medal in the 2-20 to 0-19 defeat of Clare.
[20] On 5 May 2003, Brennan was described as having his "best day ever" in a Kilkenny jersey when he scored 1-03 and won his second successive National League medal after a 5-15 to 5-14 defeat of Tipperary in the final.
[24] On 2 May 2005, Brennan came on as a substitute for Richie Power and scored two points in Kilkenny's National League final defeat of Clare.
[30] On 1 July 2007, Brennan won a seventh Leinster Championship medal after scoring a point in the 2-24 to 1-12 defeat of Wexford in the final.
Brennan was the game's top scorer with 1-05 from play and won his fifth All-Ireland winners' medal and was named man of the match after a 2-19 to 1-15 victory.
[33] Brennan won a seventh Leinster Championship medal on 6 July 2008 after scoring 2-02 from play in a 5-21 to 0-17 defeat of Wexford in the final.
[38] He later won an eighth Leinster Championship medal after a 2-18 to 0-18 defeat of Dublin, however, he was substituted in the 62nd minute after scoring only one point from play.
On 5 September 2010, Brennan lined out in his ninth All-Ireland final as Kilkenny aimed to become the first team to win five successive titles.
[42] Brennan scored a total of 4-10 after starting in all eight of Kilkenny's games, including the final defeat by Dublin, during their unsuccessful National League campaign in 2011.
He won his fourth Railway Cup winners' medal after scoring three points in Leinster's 3-18 to 1-17 defeat of Connacht in the final.
[56] Brennan began his management career at club level in 2013 when he took charge of the Lisdowney junior hurling team.
[68] His pre-season preparations were hampered by not having a full complement of players available to him, including team captain Ross King who opted to leave the panel after suffering a serious facial injury in a club game.
"[69] Laois's National League campaign saw the team qualify for the knock-out stages after a defeat of Offaly and a draw with Carlow in Division 1B.
[70] Prior to the start of the Joe McDonagh Cup, Brennan received a boost when Ross King returned to the panel.
After remaining undefeated in the group stage following three wins and a draw, Laois qualified to play Westmeath in the final on 30 June 2019.
Brennan's second season in charge saw Laois preserve their Division 1 status in the National League, in spite of losing all bar one of their group stage matches and finishing just outside the relegation play-off position.
[73] A post-season interview by Brennan with Colm Parkinson on The GAA Hour podcast caused controversy as certain "off the record" elements in which he was severely critical of the Laois County Board were broadcast in error.
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