Grianán of Aileach

Prehistoric cashel or ringfort at the entrance to the Inishowen Peninsula, County Donegal, Republic of Ireland, 10 km/6 mi northwest of Londonderry. Situated on a 240 m-/800 ft-high mound above Loughs Swilly and Foyle, the massive circular structure, 23 m/77 ft in diameter, is further protected by three earth ramparts. Dry-stone walls, 5m/17ft high and 4 m/13 ft wide at the base, incorporate four tiers of inner terraces and two passages. Built on a site of early pagan worship, its original purpose is unknown, and construction dates differ between 1700 BC and 5 BC. From the 5th century it became the seat of the O'Neill dynasty, kings of Ulster, until sacked by Murtogh O'Brien, King of Munster, in 1101. Reconstruction took place in the 1870s.

Grianán of Aileach appears on a 2nd-century map of Ireland by the Alexandrian geographer Ptolemy. In AD 450 Eoghán (Owen), founder of the O'Neill dynasty, was reputedly baptized here by St Patrick.