On the summit of Greenan Mountain is the stone fort known as the Grianán of Aileach.
From the site there are spectacular views over Lough Foyle, Lough Swilly and the surrounding countryside. The stone fort is believed to have been constructed during the 8th or 9th century as the seat of Cenél nEógain, rulers of the ancient kingdom of Aileach. The interior of the circular fort measures some 23m across and an entrance passage extends through the thickness of the massive encircling wall. This dry-stone wall stands some 5m high and incorporates a series of terraces that are accessed by stone steps.
The monument clearly has ancient origins and surrounding the stone fort are the remains of a much older hillfort that was most likely constructed around 1000BC. There is also a small stone cairn nearby that is most likely a burial monument from the Neolithic or Early Bronze Age.