They appear as low, horseshoe-shaped mounds, typically dark with charred and fire-cracked stone, and they sit in low-lying or marshy ground with a quiet persistence that has outlasted almost everything built around them. The example at Levallinree in County Mayo is one of these, a monument that belongs to a tradition stretching back roughly four thousand years.

The working theory behind fulachtaí fia, developed and refined over decades of excavation, is that they were cooking sites. A trough, usually timber-lined or cut into the earth, was filled with water, and stones heated in a nearby fire were dropped in to bring the water to a boil. Broken and heat-shattered stones were then piled to the sides, eventually forming the characteristic mound that survives today. Some researchers have proposed alternative uses, including brewing, hide-processing, or bathing, and the honest answer is that a single site may have served several purposes at different times. What almost all fulachtaí fia share is their preference for wet ground, which made the trough easy to fill and kept it topped up naturally. In Mayo, where boggy hollows and slow-draining pasture are common, conditions like these are not hard to find, and the county has a significant concentration of these monuments as a result.