They appear as low, horseshoe-shaped mounds, typically found near a water source, and they date predominantly from the Bronze Age, roughly 1500 to 500 BC. The one recorded near Fieries, a small settlement in the shadow of the Slieve Mish mountains in County Kerry, is one such survival, quiet and largely unremarked, embedded in a landscape that holds a considerable density of prehistoric activity.

The way a fulacht fia worked was straightforward but ingenious. A trough was dug into the ground and lined, sometimes with wood or stone, to hold water. Stones were heated in a fire nearby and dropped into the trough until the water boiled. The scorched and shattered stones were then raked out, and over time they accumulated into the distinctive mound that survives today. What exactly these sites were used for has been debated at length; cooking is the most widely accepted explanation, but brewing, hide-working, and bathing have all been proposed with varying degrees of plausibility. The Kerry landscape in particular is dotted with them, suggesting that whatever purpose they served, it was a repeated and practical one rather than a ceremonial rarity.