O’Brien’s Castle on Inis Oírr in the Aran Islands was built in the 14th century. The castle was taken from the O’Briens by the O’Flaherty clan of Connemara in 1582.

Because the Aran Islands were the key to controlling Galway Bay, they were contested, fortified and garrisoned by various competing powers. During the Medieval period until 1582, the Aran Islands were controlled by a branch of the O’Brien Clan from County Clare. From as early as the 13th century, there are records of payments of large amounts of wine by Galway City to the O’Brien Clan to keep the shipping routes in the area free from piracy.

The castle was taken from the O’Briens by the O’Flaherty clan of Connemara in 1582. The castle was occupied by the O’Flahertys and other clans until 1652, when the Aran Islands were surrendered to Cromwellian forces. The castle appears to have been partially dismantled at that time in favour of the Arkin Fort on Inis Mór.