Donegal's weather is honest about itself: it is a northwest Atlantic county and it rains accordingly. The driest months are April and May, and the longest days are June and July, making early summer the most comfortable time to visit. That said, Donegal in mist and low cloud has its own dramatic appeal, and the landscape looks spectacular in almost any weather.
July and August bring the bulk of Irish domestic tourism — families, summer schools, people retreating from the cities. Accommodation fills up around the coast and in popular villages, but Donegal is large enough that crowds are rarely oppressive. The Lúnasa festival and various traditional music events happen through the summer.
Late September and October are excellent for walking and for photography: the heather turns purple on the mountains, the light is dramatic, and the crowds have largely gone. Winter is cold and very wet, with short days and occasional storms that close the coast roads.