The one at Garryduff in County Mayo is a quiet example of a feature that appears almost everywhere in the Irish landscape and is still not fully understood. A fulacht fia typically survives as a horseshoe-shaped mound of fire-cracked stone and charcoal-dark earth, the accumulated debris of repeated burning and heating. The working principle involves heating stones in a fire and then dropping them into a water-filled trough, bringing the water rapidly to a boil. What exactly that boiling water was used for has kept archaeologists arguing for generations, with cooking the most widely accepted explanation, though brewing, textile processing, and bathing have all been seriously proposed.

These sites are predominantly Bronze Age in date, with most radiocarbon dates clustering between roughly 1500 and 500 BC, though some are earlier and a handful appear to be later. They tend to occur near streams or boggy ground, a practical necessity given the quantities of water the process required. Mayo has a particularly dense scatter of them, which is partly a reflection of the county's extensive bogland, where the characteristic mounds preserve well beneath the peat. The place name Garryduff, from the Irish Garraí Dubh meaning the black garden or dark enclosure, hints at the kind of low-lying, potentially waterlogged ground that would have made a site like this functional in the first place.