The Grianan of Aileach (also spelled Ailech; Irish Grianán Ailigh) is a group of historic monuments in

County Donegal, Ireland,  built on the hill of Grianán which is 244 metres high. Most writers have identified the site as being the great “royal fort” of Aileach. The main monument is that of an Irin Age stone fortress. It is generally accepted to be the seat of the Kingdom of Aileach although the true capital is now believed to lie further to the east. The kingdoms of Ulaidh and Kingdom of Oirialla were two subject kingdoms in the North under the general rule of Aileach. Whatever its true status, the Grianán was a historical centre of culture and politics during the rule of early Irish chieftains (c. 800 B.C.E.-1200 C.E.).

An Irish creation myth claims that this fortress was built by the great king Daghda of Tuatha de Dannan. The grave of the king's son Aeah was in the centre of the fortress, which had been built around it. Whether such a grave existed is unknown; there are no archaeological remains, although a nearby tumulus could be a grave marker.

By the 12th century, the Kingdom of Aileach had become embattled and lost a fair amount of territory to the invading Normans. The fortress itself was destroyed by Muirchertach Ua Briain King of Munster in 1101. By around 1177, the Normans controlled large portions of lands once held by Aileach.

(above information courtesy of Wikipedia)