From a young age he excelled at sports including Gaelic football and played for his local side Heston Gaels.
Campbell started his professional playing career in 1997 at London Irish and was part of the Under 19 World Cup winning side of 1998.
After succeeding Neil Doak for the scrumhalf position at Ulster Kieran was to enshrine himself in the Ravenhill faithful for a stunning individual try scored against Stade Français during the December 2004 Heineken Cup match.
Following in his breakthrough to the Irish national squad Campbell continued with a successful 2006–07 season where he would win a further 21 caps with Ulster.
The emergence of Isaac Boss at Ulster and his subsequent breakthrough into the Irish National side that was to allow Campbell to leave the Ravenhill setup in 2008 after seven seasons to join fellow Celtic League team Connacht.