A sweathouse, known in Irish as a tigh allais, was a pre-modern therapeutic structure used to treat ailments ranging from rheumatism to fever. The patient would crawl inside, where a fire had been lit and then raked out, and endure the heat before plunging into cold water nearby. The proximity of this example to a stream is entirely typical of the tradition.

The chamber at Greaghnaleava More is D-shaped in plan, with internal dimensions of roughly 1.5 metres north to south and 1.4 metres east to west, and a height of 1.75 metres. Its walls incline slightly inward and the roof is lintelled, meaning flat stones were laid horizontally across the top rather than corbelled or arched. The entrance, facing east, is strikingly narrow: just 0.45 to 0.5 metres wide and 0.7 metres high, which would have required anyone entering to crouch low and squeeze through. That tightness was deliberate; a small opening retained heat far more effectively than a wider door. The structure sits approximately ten metres west of the stream and around eighty metres north-east of a deserted house, the remains of which suggest a once-inhabited landscape now largely emptied. The sweathouse appears only on the 1914 edition of the Ordnance Survey six-inch map, indicating it had already passed out of common use by that point, though many such structures across Ireland are considerably older than their first cartographic appearance.