A spectacular man made construction stands on the Inishowen peninsula 7 miles south of Fahan, It consists of a terraced circular wall 17 ft high and 13 ft thick forming an enclosure 77 feet in diameter. It is not known who built it but it is thought it may have been built in the last centuries BC for rituals connected with sun worship, The ceremony would have taken place in the centre with spectators watching from the terraces.

The arrival of Christianity eventually brought an end to rituals of this kind which were repressed or taken over and modified to fit the Christian doctrine

The fort was for a time the residence of the O'Neills Kings of Ulster. Muirchertach son of Nial Glendubh became King in 919 after the death of his father in a battle outside Dublin. He used the Grainan as his base, he repulsed a force of thirty-two Viking ships in Lough Foyle. And in 926 he defeated the Vikings again on Strangford Lough in County Down, and is said to have beheaded 200 Vikings at Carlingford in County Louth in the same year. He was known as 'Muirchertach of the leather cloak' because he and his men wore leather cloaks in winter. He was killed in the battle of Ardee in 943. The Grainan was damaged by wars and the ravages of time over the centuries, but was restored in the 1970's.

See also prehistoric sites in Ireland. Also the Chronology of Ireland