Arkin's Castle (Caisleán Aircín) is a ruined thirteenth-century tower house at Killeany on the southeastern shore of Inis Mór, built by the Anglo-Norman settlers who briefly asserted control over the island in the medieval period. It is the only structure on Inis Mór that belongs unmistakably to the Norman tradition of military architecture, and it sits incongruously among the prehistoric and early Christian remains that dominate the island's heritage.
The castle is small and substantially ruined, but the remaining walls are solid and the location near the harbourfront at Killeany makes it easy to combine with the walk to Teampall Bheanáin. A plaque on the wall explains its history. The O'Brien family, who controlled the Aran Islands for much of the medieval period, held the castle after the Normans withdrew.