This is a sweathouse, the Irish equivalent of a rudimentary sauna, where a fire would be lit inside to warm the stone walls, the embers raked out, and a person crawled in to sweat out illness or joint pain. The practice was widespread across Ireland, particularly in Ulster and Connacht, but individual examples are easy to miss in the landscape, and this one at Greaghnageeragh is quieter than most.

The structure is drystone-built, meaning no mortar was used, just carefully stacked stone, and it takes a subrectangular form with slightly inclined walls and a lintelled roof, where flat stones span the top of the opening rather than forming a true arch. The interior measures roughly 1.2 metres by 1.3 metres, with a height of 1.65 metres, so an adult could just about stand upright. The entrance, facing south-west, is considerably more modest: 0.4 metres wide and 0.7 metres tall, requiring anyone entering to crouch down low. It is built into the steep south-west-facing slope of a natural rise in the ground, which would have helped retain heat and provided structural support on one side. About 60 metres to the north sits a deserted house, a reminder that this part of Roscommon once held a small community for whom this chamber was presumably a practical resource rather than a curiosity. The sweathouse appears on the 1914 edition of the Ordnance Survey six-inch map, though the structure itself is likely considerably older than that record.