Errigal is a 751-metre mountain near Gweedore in County Donegal, Ireland. It is the tallest peak of the Derryveagh Mountains and the tallest peak in County Donegal. Errigal is also the most southern and the highest of the mountain chain called the "Seven Sisters" by locals. Wikipedia
my guess is that it's used for outdoor dining, I couldn't get any closer to check it out. There is an eatery out front on the main road but it's closed.