The Castle is based on a man-made island (known as a crannóg) on Lough Oughter, and is accessible only by boat. It was built in the early 13th century by the Anglo-Normans under William Gorm De Lacy. It was soon seized by the O’Reillys (Local Irish Chieftains) who controlled it until the 16th century.

The castle and lands were granted to Captain Hugh Culme as part of the British Colonisation of Ulster, but were then seized by Irish rebels during the 1641 rebellion. 

Philip O’Reilly who was a Cavan MP and leader of the rebel forces during the Rebellion, seized control of the castle and kept it as an island fortress for the next decade. 

During this period it was mainly used as a prison after the victory of Eoghan Roe O’Neill (Famous Irish General in the Spanish Army) over the English at the Battle of Benburb in 1646. Its most notable prisoner would have been the Anglican Bishop of Kilmore, William Bedell, who was held here and is said to have died because of the harsh winter conditions in the prison.

Clough Oughter castle became the last remaining stronghold for the rebels during the Cromwell era, but sometime in March of 1653 the castle fell, during a siege, to Cromwells canons. The walls were breached by the canon and the castle was never rebuilt after this point.

In 1987, an excavation at the site uncovered at least four phases of building.

Visitors will be astounded to note the thickness of the walls which can now be seen because of the canon bombardment. The island and the castle have received considerable refurbishments making it safe to visit, and well worth the visit.