Cooney made his senior debut for Leinster on 2 September 2011, when he started against Ospreys in Swansea during the opening game of the 2011–12 Pro12. Cooney continued to make appearances for Leinster in the league, playing four more times, though each of these appearances came from the bench. Most of Cooney's appearances came while Leinster's front-line players were away at the 2011 World Cup, though he also scored his first try for the team later in the season in a win over Benetton Treviso. Cooney made his European debut for the side in the final of the 2011–12 Heineken Cup, coming on as a replacement scrum-half in Leinster's 42–14 win over Irish rivals Ulster and winning a medal in the process.