O'Brien's Castle (Inisheer)

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O'Brien's Castle, also known as Furmina Castle, is a medieval tower house on Inisheer, the northern island of the Aran Islands in County Galway, Ireland. Built about 1400 by Clann Teige, a branch of the O'Brien family, it sits on one of the island's highest points. The site stands on the old ringfort called Dún Formna, which dates from ancient times (roughly 5th century BC to 1st century AD).

The castle changed hands to the Ó Flaithbhearthaigh (O'Flahertys) around 1582. It was captured and slighted by Oliver Cromwell's army in 1652 during the Cromwellian conquest of Ireland. The building is a rectangular donjon (keep). The doorway originally opened on the first floor and was reached by external wooden stairs. Inside, the first floor housed a great hall, with mural stairs up to the parapet. The ground floor served as a basement with three vaulted chambers. A corbel with a carved human face decorates the bartizan (a small projecting turret). O'Brien's Castle is a National Monument of Ireland.