They appear as low, horseshoe-shaped mounds, typically beside a stream or boggy hollow, and for decades their purpose was debated. The prevailing interpretation now is that they were cooking sites, probably Bronze Age in date, where stones were heated in a fire and dropped into a water-filled trough to bring it to the boil. The characteristic crescent shape comes from the gradual accumulation of cracked and fire-shattered stone, discarded after each use. One such site lies at Coolcashla in County Mayo, quietly occupying its place in a landscape that has seen continuous human activity for millennia.

The fulacht fia as a monument type belongs broadly to the Bronze Age, roughly 2000 to 500 BC, though some examples have produced radiocarbon dates stretching outside that range. The burnt mounds that define them are often surprisingly substantial when examined up close, built up over repeated episodes of use rather than a single event. Some archaeologists have suggested alternative functions beyond cooking, including hide preparation, textile working, or even bathing, and the debate has never been entirely settled. What is consistent across Irish examples is the association with water and the reliance on heat-fractured stone, the remnants of which give these sites their unmistakable profile. The Coolcashla example represents one node in that widespread, if little-celebrated, network of prehistoric activity in the west of Ireland.