Brennan was announced as one of the new recruits to Nigel Carolan's Connacht academy ahead of the 2015–16 season. The move to the Connacht academy also saw him rejoin Galwegians at senior level, having played youth rugby for the club. In his first season in the academy, Brennan featured regularly for Galwegians in the All-Ireland League and also played for the Connacht Eagles, the province's second tier side. He began training with the first team and was named in a senior squad for the first time for the 2015–16 Pro12 game against Ulster on 1 April 2016, although he did not feature from the bench. Brennan was one of four academy players who trained with Connacht during the season without getting capped for the senior team. When the team qualified for the Pro12 final however, the rest of the squad paid to bring the four to the game in Edinburgh in recognition of their contribution.