The typical form is a horseshoe-shaped mound of burnt and shattered stone, dark with charring, curving around what was once a water-filled trough. One such site sits at Levallinree in County Mayo, quietly occupying its patch of ground as it has done for somewhere between three and four thousand years.

The name fulacht fia, loosely translated, refers to a cooking place, and the leading interpretation is that these sites were used for boiling water by heating stones in a fire and dropping them into a wooden or clay-lined trough. The stones crack and splinter with the thermal shock, and over repeated use the discarded fragments accumulate into the distinctive mound that survives today. Most examples date to the Bronze Age, roughly 1500 to 500 BC, though some have produced earlier or later dates. They tend to appear near water sources, on low-lying or marshy ground, which suits both the practical need for a water supply and the conditions that have helped preserve them. Alternative theories about their function have included sweat houses, textile processing, and brewing, and it is likely that the category covers a range of activities rather than a single uniform practice. The Mayo example at Levallinree fits this broader pattern of a monument type found in virtually every county, often in clusters, sometimes in isolation, rarely with any surface drama to suggest what once went on there.