The ruins of the Franciscan Abbey, founded in 1474 by Hugh O'Donnell, are a five-minute walk south of town at a spectacular site perched at the end of the quay above the Eske River, where it begins to open up into Donegal Bay. The complex was burned to the ground in 1593, razed by the English in 1601, and ransacked again in 1607; the ruins include the choir, south transept, and two sides of the cloisters, between which lie hundreds of graves dating to the 18th century. The abbey was probably where The Annals of the Four Masters, which chronicles the whole of Celtic history and mythology of Ireland from earliest times up to the year 1618, was written from 1632 to 1636. The Four Masters were monks who believed (correctly, as it turned out) that Celtic culture was doomed by the English conquest, and they wanted to preserve as much of it as they could. At the National Library in Dublin, you can see copies of the monks' work; the original is kept under lock and key.
Off N15, at the end of the quay, Donegal Town, Co. Donegal, Ireland