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.
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.
[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.
[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.