Sweathouses, known in Irish as "tigh allais", were essentially native saunas, used for centuries as folk remedies for rheumatism and other ailments. A fire would be lit inside the stone chamber, the embers raked out, and the user would crawl in and sweat it out before plunging into a nearby stream or pool. The one at Mountallen is a small, circular, drystone-built chamber, roughly 1.6 metres in internal diameter, with a lintelled entrance at the north-east just wide enough to squeeze through at around 0.4 metres across and 0.4 metres high. The roof has since collapsed, and the whole structure sits within an earthen mound measuring about 3.5 metres north to south and 2.5 metres east to west.

What makes this particular example somewhat poignant is how close it came to vanishing from the record altogether. It appears on the 1914 edition of the Ordnance Survey six-inch map, but not, it seems, on earlier editions, which suggests it was either overlooked or recorded belatedly. Sweathouses are most commonly found in Ulster and north Connacht, and examples in Roscommon are not especially numerous, which gives this one a quiet regional significance. The structure is drystone-built, meaning no mortar was used, the stones simply laid and fitted against one another, a technique that requires considerable skill to do well and that has left many such buildings surprisingly durable despite centuries of neglect. A house sits just to the east of the site, which may have contributed to its survival, the mound perhaps mistaken for a natural feature of the ground rather than something worth clearing.