It looks, at first glance, like a collapsed field shelter or a forgotten corner of a farm wall. In fact it is a sweathouse, a type of indigenous Irish sauna used for therapeutic purposes, and one of hundreds of such structures that once dotted the Irish countryside, particularly across Ulster and Connacht, before the practice quietly died out.

Sweathouses, known in Irish as teach allais, operated on a principle of intense dry heat. A fire would be lit inside the chamber, the ashes raked out, and a person would then crawl in through the low entrance and sit in the residual warmth, often for the relief of rheumatism or other ailments. The structure at Ardnaglass reflects this function precisely in its proportions: the interior measures just 1.6 metres along its longest axis and 1.1 metres in height, barely enough to crouch inside. The walls are built from un-mortared limestone flags and rubble laid in rough courses, a technique known as drystone construction, and the roof is corbelled, meaning the limestone slabs are laid in overlapping rings that close inward to form a self-supporting vault, without mortar or timber. Externally, the whole thing is covered in sod, which would have helped retain heat. The entrance, now a ragged gap 0.8 metres wide and 0.7 metres high in the northwest wall, was originally sealed with a lintel stone, though that has since collapsed. The structure dates to after 1700, placing it within the period when sweathouse use is best documented in Irish folk tradition and travellers' accounts.

The site sits in gently undulating pasture on a moderate slope, the kind of unassuming agricultural land where such structures were typically built close to a water source, since users would plunge into a nearby stream or pool after the heat. Whether a stream once ran conveniently close here is not recorded, but the general pattern holds across comparable sites elsewhere in the region.