Donegal consists chiefly of low mountains, with a deeply indented coastline forming natural loughs, of which Lough Swilly is the most notable. The Slieve League cliffs are the second highest sea cliffs in Europe, while Donegal's Malin Head is the most northernly point on the island of Ireland.

The climate is temperate and dominated by the Gulf Stream, with cool 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.