McCullen then joined Leinster Rugby in 2001 as Ireland moved away from the All-Ireland League and towards the Celtic league. That season he also played for Ireland sevens in the 2001 Rugby World Cup Sevens. He had 60 caps for Leinster and gained an international 15s cap for the game between Ireland and Samoa on 20 June 2003. His international career was halted by injury, before he was signed by Toulouse. McCullen was in the starting Toulouse team, but his time at Toulouse was once again thwarted by injury. He then joined London Irish until 2008. McCullen returned to Ireland and coached his club side Lansdowne back to division one as a forwards player/coach.