The one at Derreen in County Clare is no exception. A fulacht fia typically survives as a low, horseshoe-shaped mound of fire-cracked stone and dark, charred earth, the accumulated debris of repeated episodes of water heating. The accepted method involved dropping stones, made intensely hot in a nearby fire, into a water-filled trough until the water boiled. What that boiling water was used for, however, has been debated for decades.

For most of the twentieth century, the standard interpretation held that these sites were Bronze Age cooking places, used to boil large joints of meat. More recent experimental archaeology has complicated that picture, with researchers demonstrating that fulachtaí fia could equally have served as brewing vats, bathing facilities, or textile-working sites. The burnt mounds that characterise them are essentially the waste heaps left behind when exhausted, shattered stones were cleared away from the trough. Most date to the Bronze Age, broadly between 1800 and 800 BC, and they tend to cluster near water sources, a detail that reflects the central role water played in whatever process was being carried out. The Derreen example sits within a landscape in County Clare that contains numerous such monuments, a county whose boggy ground has helped preserve what elsewhere has long since vanished.