The original county constituency comprised the eastern part of County Antrim, being carved out of the former Antrim constituency. From 1885, East Antrim consisted of the baronies of Belfast Lower and Glenarm Upper, that part of the barony of Antrim Upper not in the constituency of South Antrim, that part of the barony of Antrim Lower not in the constituency of Mid Antrim, that part of the barony of Belfast Upper consisting of the parish of Ballymartin and the parish of Templepatrick apart from the townland of Ballyutoag, and the town of Carrickfergus.