A small but elegant 15th-century tower house, built by the O'Briens of Thomond on the highest point of Inis Oirr — about 60 metres above the sea, with views in every direction over the patchwork of stone walls below. The castle sits inside the much older earthen ringfort of Dún Formna, so the strategic value of the site goes back centuries before the tower was built.
The structure itself is roofless and partly reduced, but the main vault and several window embrasures remain, and you can scramble up to walk around the wall-walk on a clear day. A National Monument since the 1880s. The climb from the pier takes about twenty minutes by the road and another five up the path. On a fine day this is the best viewpoint on the island; in low cloud it becomes a brooding silhouette visible from anywhere on Inis Oirr.