Brennan joined the St. Lachtain's club at a young age and played in all grades at juvenile and underage levels. He enjoyed championship success in the under-21 grade before eventually joining the club's top adult team.
Brennan first played for Kilkenny as a member of the minor team during the 2013 Leinster Championship. He made his first appearance for the team on 4 May 2013 when he lined out in goal in Kilkenny's 7–16 to 1–09 defeat of Kildare. On 7 July 2013, Brennan won a Leinster Championship medal after again lining out in goal in Kilkenny's 1–18 to 0–08 defeat of Laois in the final. Brennan was again eligible for the minor grade in 2014. He won a second successive Leinster Championship medal on 6 July 2014 following Kilkenny's 2–19 to 2–10 defeat of Dublin. On 7 September 2014, Brennan lined out in goal when Kilkenny faced Limerick in the All-Ireland final. He ended the game with a winners' medal following the 2–17 to 0–19 victory. Brennan subsequently progressed onto the Kilkenny under-21 team during the 2015 Leinster Championship. He made his first appearance for the team on 2 June 2015 when he lined out in goal in Kilkenny's 4–12 to 2–16 defeat of Dublin in the quarter-final. On 8 July 2015, Brennan was again in goal when Kilkenny suffered a 4–17 to 1–09 defeat by Wexford in the Leinster final. On 5 July 2017, Brennan won a Leinster Championship medal with the under-21 team after lining out in goal in Kilkenny's 0–30 to 1–15 defeat of Wexford in the final. On 9 September, he was again in goal for Kilkenny's 0–17 to 0–11 defeat by Limerick in the All-Ireland final. Brennan ended the season by being named on the Bord Gáis EnergyTeam of the Year.
Senior
Brennan was drafted onto the Kilkenny senior team at the start of the 2017 season. He remained as a non-playing substitute during the National League and Championship campaigns. On 25 February 2018, Brennan made his first appearance for the Kilkenny senior team when he lined out in goal in a 2–22 to 2–21 defeat of Tipperary in the National League. On 8 April 2018, he was an unused substitute when Kilkenny defeated Tipperary by 2–23 to 2–17 to win the National League title. Brennan was selected on the bench when Kilkenny faced Galway in the Leinster final on 1 July2018. He remained as an unused substitute for the 0-18 apiece draw. He remained on the bench for the replay a week later, which Kilkenny lost by 1–28 to 3-15. On 11 May 2019, Brennan made his Leinster Championship debut in Kilkenny's 2–23 to 1–21 defeat of Dublin. On 30 June 2019, he was an unused substitute when Kilkenny suffered a 1–23 to 0–23 defeat by Wexford in the Leinster final.
