O'Neill was the second but eldest surviving son of Edward O'Neill, 2nd Baron O'Neill, and his wife Lady Louisa Katherine Emma (née Cochrane). Hugh O'Neill, later Baron Rathcavan, was his younger brother.
O'Neill fought in the First World War as a captain in "A" Squadron 2nd Life Guards. He was killed in action at Klein Zillebeke ridge on 6 November 1914, aged 38, the first MP to be killed in the conflict. He is commemorated on the Menin Gate in Ypres.[5] O'Neill is also commemorated on Panel 8 of the Parliamentary War Memorial in Westminster Hall, one of 22 MPs that died during World War I to be named on that memorial.[6][7] O'Neill is one of 19 MPs who fell in the war who are commemorated by heraldic shields in the Commons Chamber.[8] A further act of commemoration came with the unveiling in 1932 of a manuscript-style illuminated book of remembrance for the House of Commons, which includes a short biographical account of the life and death of O'Neill.[9][10]
O'Neill and his wife had five children; three boys and two girls. Their youngest child, Terence, was less than two months old at the time of his father's death.[12] Their eldest son Shane succeeded his grandfather in the barony in 1928, while their third son Terence O'Neill was Prime Minister of Northern Ireland between 1963 and 1969.