Dún Aengus is the most famous of several prehistoric forts on the Aran Islands, of Co. Galway. Ireland. It is located on Inishmore at the edge of a 100-metre high cliff. Dún Aengus is an important archaeological site that also offers a spectacular view. It was built during the Bronze Age and dates from 1,000 B.C. or before. It has been called "the most magnificent barbaric monument in Europe." The name "Dún Aengus" meaning "Fort of Aengus" refers to the pre-Christian god of the same name described in Irish mythology.

The fort consists of a series of four concentric walls of dry stone construction. Surviving stonework is four metres wide at some points. The original shape was presumably oval or D-shaped but part of the cliff and fort have since collapsed into the sea. Outside the third ring of walls lies a defensive system of stone slabs, planted into the ground that is still largely well-preserved. These ruins also feature a huge rectangular stone slab, the function of which is unknown. Impressively large among prehistoric ruins, the outermost wall of Dún Aengus encloses an area of about 14 acres. Although clearly defensible, the particular location of Dún Aengus suggests that its primary purpose was religious and ceremonial rather than military. It may have been used for seasonal rites by the druids, perhaps involving the bonfires that could be seen from the mainland of Ireland. The location also provides a view of as much as 120 km of coastline, which may have allowed for control over a coastal trading highway.

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