O'Halloran was involved with the Connacht youth set up from a young age, and trained with the academy squad at just 16 years old. He made his debut against Olympus Madrid in the 2009–10 Challenge Cup, scoring a try 30 minutes into the game. O'Halloran made his league debut in the same season, starting against Scarlets in a 2009–10 Celtic League match on 25 April 2010. The following season, while still in the academy, O'Halloran made three replacement appearances in the 2010–11 Celtic League, but didn't feature in the Challenge Cup.