Baron O'Neill, of Shane's Castle in the County of Antrim, is a title in the Peerage of the United Kingdom. It was created in 1868 for the musical composer Reverend William O'Neill. Born William Chichester, he succeeded to the estates of his cousin William O'Neill, 3rd Viscount O'Neill, in 1855 (on whose death the viscountcy and barony of O'Neill became extinct) and assumed by Royal license the surname of O'Neill in lieu of Chichester. Lord O'Neill was the great-great-great-grandson of John Chichester, younger brother of Arthur Chichester, 2nd Earl of Donegall. The latter two were both nephews of Arthur Chichester, 1st Earl of Donegall, and grandsons of Edward Chichester, 1st Viscount Chichester (see the Marquess of Donegall for more information). Lord O'Neill was succeeded by his eldest son, the second Baron. He sat as a Conservative Member of Parliament for Antrim.
His eldest son and heir apparent, the Hon. Arthur 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 the Hon. Arthur O'Neill). He was killed in action in Italy during the Second World War. As of 2010 the title is held by his son, the fourth Baron, who succeeded in 1944. 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. 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.