They appear as low, horseshoe-shaped mounds, typically found near water, and represent the remains of ancient cooking sites, or possibly bathing, dyeing, or brewing places, used predominantly during the Bronze Age. The one at Cloontubbrid, in County Mayo, is a quiet example of this widespread but still puzzling tradition.

The standard interpretation of a fulacht fia involves a trough, dug into the ground and often timber-lined, which was filled with water and heated by dropping fire-cracked stones into it. Those shattered, heat-spent stones were raked aside after each use and gradually accumulated into the characteristic crescent mound that survives today. The Irish term itself is somewhat contested; it translates loosely as "cooking place of the deer" or "cooking place of the wild," though what exactly was being cooked, or whether cooking was even the primary purpose, remains a matter of ongoing debate among archaeologists. Experiments have shown the method to be surprisingly efficient, capable of bringing a large volume of water to a boil within minutes and sustaining it long enough to cook a substantial joint of meat.