They appear as low, horseshoe-shaped mounds, typically found near water, and are generally dated to the Bronze Age, roughly 1500 to 500 BC. The mounds are the accumulated debris of repeated burning: stones were heated in fire and dropped into a water-filled trough to bring it to a boil, and the thermally shattered fragments were raked aside after each use, building up over time into the characteristic spread of cracked and blackened rock that survives today. What they were actually used for remains genuinely contested. Cooking is the most widely accepted explanation, but brewing, bathing, and textile processing have all been proposed with varying degrees of seriousness.

The example at Ballyguin in County Mayo is one of many such sites recorded across the west of Ireland, a region where boggy, low-lying ground near streams provided exactly the conditions these sites seem to favour. Mayo as a county has a substantial concentration of Bronze Age activity, and fulachtaí fia form a quiet but persistent thread through that landscape, often surviving precisely because the wet marginal ground around them was never worth ploughing up or building over. The site at Ballyguin stands as a local instance of a practice that was, in its time, thoroughly ordinary, repeated at countless locations across the island over the course of many centuries.