Caislean Uí Bhriain (O'Brien's Castle) stands on Inis Oírr, the smallest of the Aran Islands in County Galway, occupying one of the island's highest points. Built either in the 14th century or in 1585 by the O'Brien clan of County Clare, the rectangular keep rises within an ancient ringfort called Dún Formna, believed to date from 400 BC. The castle features a late medieval hall house and offers panoramic views of the surrounding seascape; it was temporarily occupied by the O'Flaherty's of Connemara in 1582 and remains a key historical landmark on the island.
