The passage leading into the chamber at Annacarney is just under half a metre wide and barely over half a metre tall, a tight crawl of stone that opens into a corbelled room no larger than a generous wardrobe. That combination of awkward entry and enclosed interior was not an oversight; it was the whole point. Sweathouses, found at scattered locations across Ireland, were the indigenous equivalent of a sauna. A fire would be lit inside the chamber, the heat allowed to build up within the stone walls, the embers then cleared away, and a person suffering from rheumatism or other complaints would crawl inside to sweat out whatever ailed them. The nearby stream, here running just six metres away, provided the cold plunge that completed the treatment.

The Annacarney sweathouse sits tucked into the steep side of a stream gully, built partly into the valley slope so that the surrounding earth and stone form a mound roughly four metres square and two metres high. Inside, the chamber measures approximately one and a half metres in each direction, with a corbelled roof, a construction technique in which stones are laid in overlapping courses that gradually close inward to form a vault, rising to a height of about one and three quarter metres. The floor is beaten earth. The entrance passage, approached from the north-north-east, has a slanting slab roof and narrows to just forty-four centimetres across at its tightest. The structure was documented by Price in 1952, and the roof sustained some damage to its upper portion in 1967. Sweathouses of this kind are generally considered to date from the early modern or post-medieval period, though many remained in use well into the nineteenth century as folk medicine persisted long after more formal treatments became available.