The one at Kiltrassy in County Kilkenny is typical in its quiet anonymity: a low, horseshoe-shaped mound of burnt and shattered stone, dark with charcoal, sitting in damp ground near a water source. These mounds are the accumulated debris of repeated use over centuries, the cracked stones discarded each time they were heated and plunged into a water-filled trough. What the troughs were used for remains genuinely contested among archaeologists, with cooking the most widely accepted explanation, though textile processing, brewing, and bathing have all been argued seriously.

Most fulachtaí fia date to the Bronze Age, broadly speaking the period between around 2000 and 500 BC, though some were used into the early medieval period. The mechanics were straightforward: stones heated in a fire were transferred into a wooden or stone-lined trough filled with water, bringing it rapidly to the boil. The thermal shock of this process cracked the stones, rendering them useless for reheating, and so they were tossed aside. Over generations, the pile of fractured, fire-reddened stone grew into the mound that survives today. Ireland has an unusually high density of these sites, partly because the damp, boggy ground that fulachtaí fia favoured also preserved them, and partly because the burnt stone is not especially useful for later building or field clearance, so farmers tended to leave them alone.