The origins of the Grianan of Aileach fort are dated back to 1700 B.C. It is linked to the Tuatha de Danann who invaded Ireland before the Celts and built stone forts on top of strategic hills. They worshipped Dagda (the Good God) and he too is associated with the origins of Aileach. It was he who ordered the building of a stone fort to act as a burial monument to his dead son.
The round fort is built largely without mortar to remain close to the original building. The interior has three terraces and wooden structures were built against the terraces to provide accommodation. The outline of Bronze Age or Iron Age ramparts can be seen below the fort. Legend states that the giants of Inishowen are lying sleeping but when the sacred sword is removed that they will spring to life reclaiming their ancient lands.

Mr. Walter Bernard of Derry restored the stone fort or cashel in the 1870's. It is the centrepiece of the site, 23m in internal diameter with walls measuring 5m in height and approximately 4m in thickness. The dry stonewalls contain two passages within them as well as terraces along their interior which would have allowed access to their summit. It is comparable to Staigue fort in Co. Kerry.

The remains of the three earthen banks, which surround the cashel, are still visible in places. They are heather covered with possible ditches located in between. In the east northeast a gap in the banks indicates the location of an ancient road, which passed through them. Petrie, who surveyed the site in 1835 showed settings of stones, no longer visible, between this gap in the banks and the cashel.

The tumulus, located midway between the inner and middle bank in the southeast, is no longer visible due to the covering of heather. It consisted of a low mound of stones. The well is located to the south of the cashel between the outer and middle bank. It is thought that St. Patrick visited the site in the 5th century and baptised the local chieftain, Eoghan (from whom Inis Eoghan gets its name), here.

Two theories are put forward as to its function, whether it was a defensive or ceremonial site. We do know that it was the royal site of the Northern Uí Neill between the 5th and 12th centuries A.D. According to the Annals of the Four Masters Murtagh O'Brien, King of Munster, destroyed it in 1101 A.D.

Take the main N13 road to Bridgend and the road left to Burnfoot, Buncrana and the Inishowen 100 Drive. On the approach to the village of Fahan, St. Mura's Cross and Slab are situated in the graveyard of the old Abbey, which lies alongside the main Buncrana to Derry Road. This monument is well signposted.