1264-1800: A Topographical Dictionary of Ireland by Samuel Lewis discusses the administrative history of Antrim. It is uncertain when Antrim was made a County and given representation as such in Parliament. Something like the modern arrangements seem 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 Borough (from 1666), Belfast (1613), Carrickfergus (1326), Lisburn (1661) and Randalstown (1683).
The Lord Lieutenant wrote to the Sheriff of Antrim on 2 November 1665 recommending Poyntz as the successor of Skeffington, who had inherited a peerage in September. In the absence of evidence to the contrary it is assumed that, in this period, such a recommendation was tantamount to election.