The Grianán of Aileach is a circular stone fort on top of Greenan Mountain at Burt, at the southern entrance to Inishowen. Its wall is about 4.5 metres thick and 5 metres high, with three terraces linked by steps on the inside and two passages within the wall itself. It was probably built in the 6th to 8th centuries as the seat of the Kingdom of Ailech, on a hilltop used long before, and what you see now is largely a reconstruction of 1870.
The name is usually explained as "the stone palace of the sunny view", and the view is the point. From the walls you look down over Lough Swilly and Lough Foyle and across Donegal, Derry and Tyrone. It is 11 kilometres north-west of Derry. The site is a national monument owned by the state, and a natural first stop on the Inishowen 100.