It is, in fact, a sweathouse, one of Ireland's more curious vernacular structures. Sweathouses were essentially small stone saunas, used in rural Ireland well into the eighteenth and nineteenth centuries as a folk remedy for rheumatism and other ailments. A fire would be lit inside to heat the stone, then raked out, and a person would crawl in and sweat for a period before plunging into cold water nearby. This example on Inishmaine is unusually elaborate in its construction.

The structure is trapezoidal in plan, roughly 5.5 metres north to south and 5.3 metres east to west, and stands 3.2 metres high. What sets it apart is its interior arrangement: two separate chambers, running west to east and sitting side by side, each accessed from the west through its own acutely-pointed doorway just 0.7 metres wide and 1.2 metres high. The floors are slightly raised above ground level on spaced stone slabs, a practical detail that would have kept a user clear of ash and debris. Both the north and south chambers are nearly identical in size, each about 3.1 metres long, under a metre wide, and 1.5 metres high, roofed with pointed arches of dry or mortared stone. The paired arrangement, with two independent chambers within a single mass of masonry, is not the typical form for a sweathouse and suggests either community use or a structure built with some deliberation and resource behind it. Approximately 300 metres to the northwest lies Inishmaine Abbey, placing this structure within a landscape that has been shaped by both ecclesiastical and everyday rural life for centuries.