The one recorded at Carrigeenwood in County Kerry is typical in that regard: a low, horseshoe-shaped mound of fire-cracked stone, dark and waterlogged, sitting quietly in the landscape with little to announce its age or purpose. Most date to the Bronze Age, somewhere between 1500 and 500 BC, and the basic mechanics are well established. A trough, often timber-lined, was filled with water; stones were heated in a nearby fire and dropped in to bring the water to a boil. What exactly that boiling water was used for is where the argument begins, and has never quite ended.

The cooking hypothesis has dominated since the archaeologist M. J. O'Kelly demonstrated in the 1950s that a fulacht fia could efficiently cook a joint of meat wrapped in straw. But brewing, bathing, and textile processing have all been proposed with varying degrees of conviction. The cracked and shattered stones that accumulate into the characteristic mound are the byproduct of repeated heating and quenching; once a stone had fractured beyond use, it was simply discarded to the side, and over centuries of intermittent activity those discards built up into the low banks that survive today. Carrigeenwood, like most of its counterparts, sits near a water source, which was a practical necessity regardless of what was actually being cooked, brewed, or steeped.