Dún Aonghasa is the most famous of several prehistoric forts on the Aran Islands, of County Galway, Ireland. It is located on Inishmore at the edge of an approximately 100 metre high cliff.A popular tourist attraction, Dún Aonghasa is an important archaeological site that also offers a spectacular view. It is not known when Dún Aonghasa was built, though it is now thought to date from the Iron Age.T. F. O'Rahilly surmised that it was built in the second century B.C. by the Builg following the Laginian conquest of Connacht.
Dún Aonghasa has been called "the most magnificent barbaric monument in Europe." The Proto-Celtic name, meaning "Fort of Aonghas", refers to the pre-Christian god of the same name described in Irish mythology, or the mythical king, Aonghus mac Úmhór.