They appear as low, horseshoe-shaped mounds, typically beside a water source, and are thought to date largely from the Bronze Age. One such site sits at An Choill Mhór in County Kerry, a placename that translates roughly as "the great wood", suggesting a landscape that may once have looked quite different from whatever surrounds the mound today.

The mechanics of a fulacht fia are fairly straightforward, even if their precise purpose remains debated. Water was collected in a timber-lined trough sunk into the ground, and stones were heated in a nearby fire before being dropped into the trough to bring the water to a boil. The shattered, fire-cracked stones were then discarded to the sides, building up over repeated use into the characteristic crescentic mound that survives. What this boiling water was actually for has kept archaeologists busy for decades. Cooking is the traditional explanation, and experimental archaeology has shown it works well enough for boiling meat. But brewing, textile processing, and bathing have all been proposed with varying degrees of conviction. The Kerry example at An Choill Mhór is one of a dense concentration of such sites in Munster, where the wet, low-lying ground provided exactly the kind of reliable water supply these features seem to require.