Derg Castle, sometimes also called Castlederg Castle, lies on the north bank of the river Derg, next to the village of Castlederg, in County Tyrone, in Northern Ireland.
A castle is first mentioned at Castlederg in 1497. It was probably in the style of a square Irish tower house and was strategically positioned to command a ford on the River Derg. The castle changed hands several times in the late 15th and early 16th centuries during the contests between the O'Neills and O'Donnells for control over this area.
In 1610 under the scheme for the Plantation of Ulster, this area was granted to Sir John Davies. The existing Derg Castle was reconstructed into a bawn with 3 open flankers. During that time Davies also built Kirlish Castle and connected them both with a straight causeway which was 7 miles long.
During the Irish rising of 1641, Derg Castle was a place of refuge for the local settler community. Apparently it was of considerable strength because a small garrison was able to withstand a siege by a much larger force, led by Sir Phelim O'Neill, before being forced to abandon the castle after running out of ammunition.
In 1689, during the Williamite War, Derg Castle was again garrisoned by the settler population, but surrendered to James II's forces on being promised that their lives would be spared. After this the castle appears not to have been used anymore and fell to ruin.
At present the tower house has disappeared. The north wall with its 2 flankers is the best preserved. The walls and flankers on the south side have largely disappeared due to erosion by the river. Of any buildings inside the bawn almost nothing survived.
Derg Castle is situated in a village park and freely accessible. A nice rural castle although there is not much architectural detail to be seen.