It measures roughly 3.5 metres north to south and 3.2 metres east to west, rising from about 1.4 metres on the uphill western side to 2.6 metres on the lower eastern face. That asymmetry is simply the slope doing its work. What makes it more than a lump of earth is what it was built for: a sweathouse, the Irish equivalent of a rudimentary sauna, where heated stones would be packed into a small stone chamber, the fire raked out, and a person crawled inside to sweat out rheumatism, fever, or whatever else ailed them. The entrance here faces east, though both the entrance passage and the chamber behind it have since collapsed, leaving the interior inaccessible.

Sweathouses, known in Irish as teach allais, are found across the northern counties of Ireland and are thought to have been in use from at least the early medieval period, though many remained in local use well into the nineteenth century. This particular example at Gubnaveagh does not appear on the earlier editions of the Ordnance Survey six-inch maps, surfacing only in the 1907 edition, which suggests it had already passed out of active use or recognition by the time earlier surveyors came through. By that point it was recorded near a house roughly 85 metres to the south-west and a road about 60 metres to the west, placing it at the edge of a small rural community rather than in complete isolation. The fact that it survived at all, even in collapsed form, is partly a matter of its construction, a corbelled or dry-stone chamber built to retain heat, and partly the accident of being on a slope that offered little agricultural incentive to clear it away.