decayed seaport town of Ireland, county Antrim, situated on a bay opposite Rathlin island. The town is well-built, consisting of two parts, about a quarter of a mile asunder, and connected by a fine avenue. Upwards of L150,000 were expended upon the harbour and pier, but the violence of the sea overthrew the pier and filled the harbour with sand, so that Ballycastle is now little more than a fishing village. Pop. in 1851, 1669.