Cullen won a Leinster Schools Senior Cup medal in 1995 as well as 1996, when Blackrock beat a Newbridge College team which included Geordan Murphy.

[3] Cullen began his career with Leinster by representing them at schoolboy level in 1995 before moving on to their U20 team where he won seven caps.

He also made five appearances on the 'A' team and finally debuted during the interprovincial and Heineken Cup matches of the 1998/99 season.

[5] He was also on the losing side in the 2007 Heineken Cup final, when London Wasps defeated Leicester at Twickenham Stadium.

[8] as of 17 March 2024[update][9] After retiring from playing in 2014, Cullen remained at Leinster as a forwards coach.