O'Hanlon was born on 8 October 1965 in Carrickmacross, County Monaghan, the son of politician and doctor Rory O'Hanlon and Teresa Ward.[citation needed] He has five siblings.[citation needed] The episode of Who Do You Think You Are? which aired on 6 October 2008 revealed that O'Hanlon's paternal grandfather, Michael O'Hanlon, was a medical student at University College Dublin who had joined the Irish Republican Army (IRA) during the Irish War of Independence and was a member of Michael Collins's squad which assassinated British secret service agents on the morning of Bloody Sunday. Details of his grandfather's activities survive in UCD Archives, as well as Blackrock College. It also transpired that, on his mother's side, he is a close relative of Peter Fenelon Collier.