 Its name simply means "fort" in Scottish Gaelic (for more information, see "dun"), but the fort itself has been lost - old maps show it on Gob an Dùin (), which is at the seaward end.Though Dùn is almost joined to Hirta at Ruiaval, the two islands are separated by Caolas an Dùin (the straits of Dùn).This prevents Soay sheep from Hirta straying onto the island, and thus it is more vegetated.