Found in their hundreds across the country, they typically consist of heaps of fire-shattered stone accumulated beside a water source, and their precise purpose has been debated for decades. Cooking, bathing, industrial processing, and ritual use have all been proposed, and the answer may well differ from site to site.

The example at Ballyclogh, in County Wicklow, came to light during road improvement works on the N11 and was excavated by archaeologist Yvonne Whitty. The mound itself measured fifteen metres by ten metres, a substantial spread, and beneath it lay a central trough lined with wattle, the woven wooden panels familiar from other prehistoric wetland contexts. The basic logic of such features is well understood: stones were heated in a fire, dropped into a water-filled trough to bring the water rapidly to the boil, and the spent, fractured stones were then raked aside into the accumulating mound. What made the Ballyclogh site particularly interesting was what lay beneath the trough itself. A layer of unburnt stones, interpreted as a working platform, underlay the main feature, and two pits were identified below that again. These pits may belong to an earlier phase of activity on the spot, suggesting the location was used, perhaps repeatedly, before the burnt mound as excavated was ever constructed.