Donegal Donegal Donegal is the small town that gave its name to the county, at the head of Donegal Bay, at the mouth of the River Eske. Around The Diamond are Donegal Castle, of the O'Donnells, and the ruins of the Franciscan convent on the estuary. It is the base for the Slieve League cliffs, about 50 km away, and for Glenveagh National Park in the north of the county.