From the outside it looks like little more than a grass-covered mound, the kind of earthen lump that rural Ireland has in abundance. Step closer, though, and a narrow opening barely half a metre wide and less than a metre tall reveals itself on the west-south-west face, the only way in or out of a structure that was once used as a kind of early Irish sauna.

Sweathouses, known in Irish as tigh alluis, were used for therapeutic sweating, typically involving a fire lit inside to heat the stones, the embers cleared out, and bathers crawling in through the low entrance to sit in the residual heat. They are found mostly in the northern half of Ireland, though examples like this one in Galway show a wider distribution. The Ballydonnellan example is built in drystone, meaning the walls are constructed without mortar, relying entirely on the careful placement of stone. The interior chamber is wedge-shaped in plan, narrowing from about two and a half metres at its base to under a metre at roof level, with the roof formed by corbelling, a technique where successive courses of stone are each laid slightly inward until they meet at the top. The walls stand between 1.7 and 1.9 metres high inside. Externally, the whole structure is encased in a grass-covered earthen mound roughly eight and a half metres across and one and a half metres high, giving it that inconspicuous, almost geological quality that makes so many ancient Irish monuments look, at first glance, like natural features of the landscape.