O'Neill was born and raised in London, the youngest son of Irish parents, Eoghan and Theresa "Terry" O'Neill who were originally from County Kildare and County Dublin. O'Neill had one sister, Siobhán, and one brother, Shane. He was a former pupil of the London Oratory School, a Roman Catholic school in Fulham, in London, where he was remembered as cheerful, well-behaved and outgoing.
O'Neill was born and raised in London, the youngest son of Irish parents, Eoghan and Theresa "Terry" O'Neill who were originally from County Kildare and County Dublin. O'Neill had one sister, Siobhán, and one brother, Shane. He was a former pupil of the London Oratory School, a Roman Catholic school in Fulham, in London, where he was remembered as cheerful, well-behaved and outgoing.