Explore Donegal South Donegal Southwest Donegal Central Donegal The Rosses Gweedore and Tory Island The Derryveagh Mountains and Glenveagh The north Donegal coast The Fanad Peninsula Letterkenny and around The Inishowen Peninsula Share Off the N13 Letterkenny–Derry road lies the ancient fort known as the Grianán Ailigh. It dates from 1700 BC, and is thought to be linked to the Tuátha Dé Danann, pre-Celtic invaders. It was sufficiently significant to be included by Ptolemy, the Alexandrian geographer, in his second-century AD map of the world, and was the base of various northern Irish chieftains. Here, in 450, St Patrick is said to have baptized Eoghán, the founder of the O’Neill clan that ruled the kingdom of Ailigh for more than five hundred years. In the twelfth century, the fort was sacked by Murtagh O’Brian, King of Thomond, in retribution for a raid on Clare, and a large amount of its stone was carried away. Today’s impressive building was largely reconstructed in the 1870s by Walter Bernard from Derry and is the only remaining terraced fort in Ireland. It’s enclosed by three earthen banks, but its most stunning asset is the view across the primordial jumble of mountains and hills far away to the west and the loughs to each side of Inishowen immediately to the north.