 O'Neill was the third son of William O'Neill, 1st Baron O'Neill (who was born William Chichester), and his first wife, Henrietta Torrens, daughter of Hon.Robert Torrens, a Judge of the Irish Court of Common Pleas.O'Neill was educated at Harrow School and at Brasenose College, Oxford and served as a Major in the 4th Battalion, Royal Inniskilling Fusiliers.