Deegan joined the O'Loughlin Gaels club at a young age and played in all grades at juvenile and underage levels before eventually joining the club's top adult team in the Kilkenny Senior Championship. On 30 October 2016, Deegan lined out at midfield when O'Loughlin Gaels faced Ballyhale Shamrocks in the Kilkenny Senior Championship final. He scored a point from play and ended the game with a winners' medal following a 0-19 to 1-12 victory. After being drafted onto the Kilkenny senior team as a member of extended training panel during the 2015 season, Deegan didn't make his first appearance for the team until 12 February 2017 when he came on as a 52nd-minute substitute for Ollie Walsh in a 1-15 to 0-17 defeat by Waterford in the 2017 National Hurling League. He made his Leinster Championship debut on 10 June 2017 when he scored a point from midfield in a 1-20 to 3-11 defeat by Wexford. On 8 April 2018, Deegan lined out at left corner-back when Kilkenny faced Tipperary in the National League final. He ended the game with a winners' medal following the 2-23 to 2-17 victory. Deegan was again selected at left corner-back on 1 July 2018 when Kilkenny drew 0-18 apiece with Galway in the Leinster final.[16] He retained his position for the replay a week later, however, Kilkenny suffered a 1-28 to 3-15 defeat. On 30 June 2019, Deegan lined out at left wing-back when Kilkenny suffered a 1-23 to 0-23 defeat by Wexford in the Leinster final.