Inside it, however, is something rather specific: a corbelled drystone chamber just under two metres across, sealed by a narrow doorway less than a metre high. This is a sweathouse, an Irish vernacular equivalent of a sauna, built to generate intense dry heat by burning peat or wood inside the chamber, raking out the embers, and then crawling in to sweat out fevers, rheumatism, and other ailments. They are found across Ulster and Connacht, typically near water, and the stream running alongside this one at Gubnaveagh was almost certainly part of the treatment, offering a cold plunge once the sweating was done.

The structure at Gubnaveagh is a subcircular mound measuring roughly 3.25 metres north to south and 2.6 metres east to west, rising to a maximum height of about 3 metres at the eastern side, where the doorway opens. The chamber within is circular, built using the corbelling technique, in which each successive ring of unmortared stones is laid slightly inward until the courses close overhead without any keystone or mortar to hold them. Internally the space measures 1.85 metres in diameter and 2.15 metres in height, tight but functional. The east-facing doorway is 65 centimetres wide and just a metre tall, meaning entry requires a deliberate stoop. The site was recorded by Weir in 1980 and appeared on the 1907 edition of the Ordnance Survey six-inch map, which places a road roughly 50 metres to the north and a house about 100 metres to the south, suggesting it was once part of a working rural landscape rather than any kind of ceremonial or monastic setting.