He was returned to the Irish House of Commons for Randalstown in 1760, a seat he held until 1783, and then represented County Antrim between 1783 and 1793.
[2] In February 1782, he had been a delegate for Co. Antrim to the Ulster provincial Volunteer convention in Dungannon, and in November 1793 one of the five Co. Antrim delegates to the national Volunteer convention in Dublin that sought to build on the legislative independence secured for Ireland the previous year, with parliamentary reform.
[3] In 1793, raised to the Peerage of Ireland as Baron O'Neill, of Shane's Castle in the County of Antrim.
[4] In 1795 he was further honoured when he was made Viscount O'Neill, of Shane's Castle in the County of Antrim, in the Irish peerage.
He was killed in the Battle of Antrim during the Irish Rebellion of 1798[6][3] at the age of 58 and was succeeded in the viscountcy by his son Charles, who was created Earl O'Neill in 1800.