The one recorded at Callow in County Mayo is a typical representative of a type that remains, in many ways, genuinely mysterious. A fulacht fia generally appears as a low, horseshoe-shaped mound, often darkened by charred and fire-cracked stone, and typically found close to a source of water. The mound is the accumulated debris of repeated use over many generations.

The sites date mostly to the Bronze Age, roughly 1500 to 500 BC, though some examples are older or later. The working principle, as understood from excavated examples, involved heating stones in a fire and then dropping them into a water-filled trough, bringing the water rapidly to a boil. What exactly this process was used for remains genuinely contested. Cooking is the most commonly proposed explanation, and experiments have shown that a substantial joint of meat can be boiled efficiently using the method. But other researchers have argued for uses ranging from textile processing and leather working to bathing. The cracked and shattered stones, rendered useless by repeated heating and cooling, were simply discarded to the side, building up over time into the distinctive mound that survives today. The Callow example sits within a part of Mayo that, like much of the west of Ireland, contains a density of prehistoric activity that the blanket bog has preserved rather than destroyed.