They appear as low, horseshoe-shaped mounds, typically found near water, and are thought to date mainly from the Bronze Age, roughly 1500 to 500 BC. The prevailing interpretation is that they were cooking sites: stones were heated in a fire and dropped into a water-filled trough, bringing it rapidly to a boil. The mound itself is formed from the cracked and discarded stones, piled up after each use. One such site sits at Clenagh in County Clare, a quiet addition to a monument type that occurs so frequently across Ireland that it is easy to overlook how strange and persistent the practice must have been.

The name fulacht fia translates loosely from Old Irish as something like "cooking pit of the deer" or "cooking place of the wild," though the exact meaning has been debated. While cooking is the most widely accepted explanation, some researchers have proposed alternatives, including use as saunas, dyeing vats, or brewing vessels. Experimental archaeology has shown that the hot-stone method works remarkably efficiently, capable of boiling a large volume of water in under half an hour. The Clenagh example joins hundreds of recorded sites across Clare alone, a county whose boggy, water-retentive landscape provided ideal conditions for this kind of activity over many centuries.