Physically, the county is by far the most rugged and mountainous in Ulster. The county consists chiefly of low mountains, with a deeply indented coastline forming natural loughs, of which both Lough Swilly and Lough Foyle are the most notable. The famous mountains or Hills of Donegal consist of two major ranges, the Derryveagh Mountains in the north and the Bluestack Mountains in the south, with Mount Errigal at 749 metres (2,457 ft) the highest peak. The Slieve League cliffs are the sixth-highest sea cliffs in Europe, while Donegal’s Malin Head is the most northerly point on the island of Ireland.
The climate is temperate and dominated by the Gulf Stream, with warm, damp summers and mild wet winters. Two permanently inhabited islands, Arranmore and Tory Island lie off the coast, along with a large number of islands with only transient inhabitants. Ireland’s second longest river, the Erne, enters Donegal Bay near the town of Ballyshannon. The River Erne, along with other Donegal waterways, has been dammed to produce hydroelectric power. The River Foyle separates part of County Donegal from parts of both County Derry and County Tyrone.