The term refers to a type of burnt mound, typically Bronze Age in origin, found in their thousands across the Irish landscape. The usual form is a horseshoe-shaped spread of heat-shattered stone and dark, charred soil, accumulated around a pit or trough that was once filled with water. Stones were heated in a fire and dropped into the trough to bring the water to a boil, a process repeated until enough cracked and spent stone built up to form the mound visible today. What they were actually used for remains a matter of debate among archaeologists: cooking is the most widely accepted explanation, but brewing, hide-working, and bathing have all been proposed, and the evidence suits more than one possibility.

Fulachtaí fia tend to cluster near wet ground, streams, or bog margins, which provided the water supply the process required. Mayo, with its abundance of low-lying, waterlogged terrain, has a considerable concentration of them. The townland name Lack derives from the Irish leac, meaning a flagstone or flat rock, a type of place-name often associated with low, damp ground, which fits the typical setting for this kind of monument. Without more detailed site-specific records currently available, the precise dimensions, condition, and immediate landscape context of this particular example remain difficult to describe with confidence, but its presence in the record marks it as a recognised archaeological monument, protected under Irish heritage legislation.