Cloughoughter Castle, Medieval castle ruin on an island in Lough Oughter, County Cavan, Ireland.

Cloughoughter Castle is a medieval ruin on an island in Lough Oughter, featuring a round stone tower that rises three stories with walls about 5 meters thick. The compact design uses the island location as a natural defense, with the surrounding water serving as the primary barrier.

The Anglo-Norman de Lacys built the castle around 1220, and the lower two storeys date from the first quarter of the 13th century. By 1233 the O'Reilly clan held Lough Oughter and kept the castle until about 1600. After the 1610 Plantation of Ulster it was granted to Captain Hugh Culme; Philip O'Reilly seized it in 1641 and held it until Commonwealth artillery under Sir Theophilus Jones forced its surrender on 27 April 1653.

The castle's Irish name, Cloch Locha Uachtair, means 'stone castle of Loch Uachtair'. In the 18th and 19th centuries the ruin became a frequent subject for artists, and in 1852 The Dublin University Magazine described it as round and massive, unlike most Irish castles, which are square.

Reaching the castle requires a boat trip across the lake from the mainland, which only works in decent weather. Wear sturdy shoes and expect wet conditions, as landing on the island can be slippery.

The round tower design differs from the square or rectangular layouts common in other Irish castles. This unusual style made it stand out architecturally among medieval lake fortresses.