His eldest son and heir apparent, Hon. Arthur Edward Bruce O'Neill, represented Antrim Mid in the House of Commons as a Conservative from 1910 until 1914, when he was killed in action during the First World War, the first MP to die in the conflict. The second Baron was therefore succeeded by his grandson, the third Baron (the son of Hon. Arthur Edward Bruce O'Neill). He was killed in action in Italy during the Second World War. As of 2007 the title is held by his son, the fourth Baron. He is Lord Lieutenant of Antrim since 1994. As a descendant of the first Viscount Chichester he is in remainder to the barony and viscountcy of Chichester and, according to a special patent in the letters patent, the earldom of Donegall, titles held by his kinsman the Marquess of Donegall.
Two other members of the O'Neill family have been elevated to the peerage. Robert William Hugh O'Neill, 1st Baron Rathcavan, was the youngest son of the second Baron O'Neill, while Terence O'Neill, Baron O'Neill of the Maine, Prime Minister of Northern Ireland, was the youngest brother of the third Baron.