The one at Cloonnagleragh, in County Mayo, is typical in its quiet anonymity, a low horseshoe-shaped mound of fire-cracked stone sitting in the kind of damp, low-lying ground these features seem almost magnetically drawn to. That association with wet ground is no accident. A fulacht fia works, at its most basic, as a Bronze Age cooking or heating site: a trough dug into the earth, filled with water, and brought to boiling point by dropping stones heated in a nearby fire into the water. The shattered, heat-stressed stones that result are raked aside after each use, and over generations they accumulate into the characteristic mound that survives today.

The Bronze Age date places these sites roughly between 2000 and 500 BC, though some in Ireland have returned earlier readings and a handful appear to have been used into the early medieval period. The purpose of fulachtaí fia has been debated at length by archaeologists. Cooking is the most widely accepted explanation, and experimental reconstructions have confirmed that the technology works efficiently for boiling large quantities of meat. Other proposals, supported by evidence at various excavated examples, include their use for bathing, textile processing, or the brewing of ale. Without excavation, it is rarely possible to say which function, or combination of functions, a particular site served. The example at Cloonnagleragh has not, as far as the available record indicates, been the subject of detailed published investigation, which leaves it in the company of the great majority of these monuments: identified, mapped, and quietly waiting.