Kielty was born in Downpatrick, County Down, Northern Ireland, and grew up in the neighbouring village of Dundrum. He is one of three sons born to the businessman John "Jack" Kielty, who was shot dead on 25 January 1988 by the Ulster Freedom Fighters (UFF), a cover name used by loyalist paramilitary group the Ulster Defence Association (UDA). John Kielty was to have been a key witness in Central Television's defence of a libel action brought by Jim Craig, who was suing the television company over a broadcast of The Cook Report which connected him to racketeering, and is said to have ordered John Kielty's murder.

Kielty as a teenager was a talented gaelic footballer, and was even a member of the Down GAA minor footballer team for 3 years between 1987–89. He was the sub goalkeeper of the team when they won the 1987 All Ireland Minor Football Championship.