Stone ring fort at the Grianán of Aileach, on the summit of Greenan Mountain, County Donegal, Ireland.
This stone fort is thought to have been constructed during the 8th or 9th century as the seat of Cenél nEógain, ruler of the ancient kingdom of Aileach. This monument however evidently had far earlier origins, surrounding the stone fort are the remains of an earlier hill fort, which was likely constructed approximately 1000 BCE. A small Neolithic or Early Bronze Age stone cairn is also nearby.
Photos courtesy & taken by Chris Newman. Reference: National Monuments Service.