County Antrim was a constituency represented in the Irish House of Commons until 1800.
In the Patriot Parliament of 1689 summoned by King James II, County Antrim was represented with two members.
[1] 1264-1800: A Topographical Dictionary of Ireland by Samuel Lewis discusses the administrative history of Antrim.
Something like the modern arrangements seems to have originated in 1584 when the Lord Deputy Sir John Perrot divided the area into baronies.
From whatever point the county constituency existed it comprised the whole of County Antrim, excluding the parts in the borough constituencies of Antrim (from 1666), Belfast (1613), Carrickfergus (1326), Lisburn (1661) and Randalstown (1683).