It is barely the size of a large wardrobe inside, and to enter it at all you would have to crouch to little more than half a metre off the ground. That tight, low entrance is not an accident of poor construction. It is exactly the point.

Sweathouses were the Irish equivalent of a sauna, used for therapeutic sweating long before any formal medical infrastructure reached rural communities. A fire would be lit inside, the chamber heated thoroughly, the embers cleared out, and then the user would crawl in through the low opening and endure the heat. The structure at Castlegarden follows the form well documented across Ireland, particularly in Ulster and Connacht, though examples in Leinster are less common. It is roughly circular on the outside, with an external diameter of around 2.75 metres and walls approximately 0.6 metres thick. Inside, the plan becomes sub-rectangular, measuring about 1.3 metres by 1.5 metres, with a clay floor underfoot and internal walls rising to 1.3 metres before the roof begins to corbel inwards. Corbelling is a technique in which each successive course of stone projects slightly beyond the one below, gradually closing the space without the need for a keystone or mortar, and here it terminates in a small circular opening at the apex, roughly 0.44 metres by 0.6 metres, which would have served as a vent. The whole thing is dry-stone construction, meaning no mortar holds it together, only the careful placement of one stone against another. The local name for it has persisted, which is often the most reliable sign that a structure was in genuine, remembered use rather than merely surviving as an anonymous ruin.