The castle was built in the mid 13th century, replacing a still existent earthen motte to the west. Together with King John's castle in Carlingford on the opposite shore, it commanded the narrow entrance to the lough. It was initially held by the de Burghs, Earls of Ulster.
After an eventful history, it was granted to Sir Nicholas Bagnal in 1552, along with most of Mourne. The castle is a typical Norman castle with towers and a curtain wall.
Greencastle is only a few minutes drive from Cranfield Beach, and was once known as the capital of Mourne.