The example that survives at Tully in County Sligo sits quietly in a coniferous forest on the southern side of a narrow valley, tucked into a meander of a stream. From the outside it reads as little more than a dome of rubble limestone rising to about two metres, its base spreading to six metres across. The only way in is a rectangular opening on the north side, barely forty-five centimetres wide and sixty centimetres high, just large enough to squeeze through on hands and knees.

A sweathouse, in its simplest form, is a primitive sweat lodge. A fire would be lit inside to heat the stone walls, the embers raked out, and then the user would crawl in through the low entrance and endure the accumulated heat, often for the treatment of rheumatism or other ailments. The Tully example is solidly built, with rough courses of limestone bonded in lime mortar, jambs of shaped limestone blocks framing the entrance, and a corbelled roof inside, meaning the ceiling is formed by stones laid in overlapping rings that gradually close toward the centre. The internal cell measures roughly 1.8 metres by 1.7 metres, with a packed earth floor and walls faced in shaped rubble. Although the structure is now overgrown with briars, the stonework itself remains coherent. Its probable date places it in the nineteenth century: it appears on the Ordnance Survey six-inch map of 1914 but is absent from the equivalent 1838 edition, suggesting it was built sometime in the intervening decades, a period when such structures were still in occasional use across the west and north of Ireland.