As Postman’s photo explains, this was said to be the cell of the Red Priest, whose Stone lies nearby. The Red Priest is supposed to be St. Maolrubha, who was busy evangelising in these parts in the 7th century. He prophesied that the population of Strathnaver would be driven from here for their sins, and would not be able to return until his bones had been washed out to sea.
This page at the