Introduction Lough Swilly (Loch Súilí) is a sea lough stretching 40 km into County Donegal from the Atlantic Ocean. Also known as the Lake of Shadows, the lough is between 15 and 50 metres deep. It is bordered on the west by Inishowen (Inis Eoghain) – Ireland’s largest peninsula – and by the Fanad peninsula on the east. Around the lough, the beautiful Donegal coastline includes spectacular cliffs, wide sandy beaches and impressive rock formations with stunning views of the mountains.