Dún Aonghasa is the ruins of a rock fort on the island of Inis Mor, on the edge of a 100 m high cliff. The fort consists of four concentric stone walls, the original shape of which probably resembled an oval or the letter D. There is a small museum here presenting the history of the fort and its eventual functions. Nearby is a Neolithic tomb and a small open-air museum with traditional thatched huts.
It is not known exactly when Dún Aonghasa was built, although it is now believed that most of the structures date back to the Bronze Age and the Iron Age. In some places, the original parts of the walls are up to 4 m high, and in modern times they were reconstructed and the whole was added to a height of about 6 m.
The name Dún Aonghasa means "Fort Aonghas" and probably refers to the pre-Christian god of the same name, described in Irish mythology. According to another version, it is about the mythical king Aonghus mac Úmhór.