This univallate (single walled) fort is located at the eastern end of Inis Meáin and overlooks the modern harbour. Unlike the other stone forts on the islands it is roughly square in plan. This type of stone fort also occurs in the Burren and south Mayo and is generally considered to be a variant on the circular cashel. It is possible however that square forts may have appeared quite late in the development of cashels and not all may have been settlement sites. The interior of Dún Fearbhaí slopes down steeply - from the higher southern part of the interior it is possible to look out over the enclosing wall towards the harbour. This suggests that the fort was built to defend the harbour (and the island) from attack and it may also have protected a fleet anchored in the bay below. The fort is likely to have been built sometime between 700 - 900 AD.