Midfielder O'Callaghan signed for Rovers in July 2008 after a short spell back at his hometown Club Cork City in Ireland.

A trainee at Port Vale, the 29 year-old spent for years in the Potteries before moving back home to Cork to play for Cork City in 2002. 

He spent five years back in his native country before returning to England to play for Ipswich Town in January 2007 where he spent twelve months, including a loan spell at Brighton and Hove Albion before joining Cork City once more in January of last season.

Between January 2008 and his departure at the end of the 2007-08 season, he netted nine times in just seventeen appearances from midfield - something Rovers have lacked in recent seasons.