Brennan joined the St Lachtain's club at a young age and played in all grades at juvenile and underage levels. He experienced championship success in the under-21 grade before eventually joining the club's top adult team.[citation needed]
Kilkenny
Minor and under-21
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.[2]
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.[3] 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.[4]
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.[5] 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.[6]
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.[7] On 9 September, he was again in goal for Kilkenny's 0–17 to 0–11 defeat by Limerick in the All-Ireland final.[8] Brennan ended the season by being named on the Bord Gáis Energy Team of the Year.[9]
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.[citation needed]
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.[10] 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.[11] Brennan was selected on the bench when Kilkenny faced Galway in the Leinster final on 1 July 2018. He remained as an unused substitute for the 0-18 apiece draw.[12] He remained on the bench for the replay a week later, which Kilkenny lost by 1–28 to 3-15.[13]
On 11 May 2019, Brennan made his Leinster Championship debut in Kilkenny's 2–23 to 1–21 defeat of Dublin.[14] 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.[15]