The Black Fort

Dún Dúchathair or simply Dúchathair, meaning "black fort", is a large stone fort on the cliffs at Cill Éinne,, Inishmore in County Galway, Ireland. Due to erosion, it now sits on a rocky promontory that stretches out into the sea. On its outer side there are large walls, reaching 6 metres high and 5 metres wide. On the inside are the ruins of clocháns. There is also evidence of a cheval de frise protecting the entrance.Its age is unknown.

Source: Wikipedia