The one at Treanrevagh in County Mayo is typical in its quiet anonymity: a low, horseshoe-shaped mound of burnt and fire-cracked stone, sitting in boggy ground and easy to walk past without a second glance. What these mounds represent, though, is a technology repeated so consistently across Bronze Age Ireland that it amounts to something like a standard solution to a recurring problem.

A fulacht fia works on a simple principle. A trough is dug into the earth, often lined with timber or stone to hold water, and stones are heated in a nearby fire before being dropped into the trough to bring the water to a boil. The shattered, heat-fractured stones are then raked aside, and over time that discarded material accumulates into the characteristic mound that survives today. Bronze Age communities used these sites for cooking, and possibly also for textile processing, bathing, or other activities requiring large quantities of hot water. The Treanrevagh example sits within a part of Mayo where bogland has preserved such monuments particularly well, the waterlogged conditions that made these sites useful in prehistory being the same conditions that have kept the evidence intact for three or four thousand years.