They appear as low, horseshoe-shaped mounds, typically found near water, and are thought to date from the Bronze Age, roughly 1500 to 500 BC. The prevailing theory holds that they were cooking sites, where stones were heated in fire and then dropped into a water-filled trough to bring it to the boil. The stones, repeatedly cracked by thermal shock, were discarded into the surrounding mound over time, which is why these sites tend to be composed largely of shattered, fire-blackened rock. One such site sits at Knockmore in County Mayo, a quiet addition to a prehistoric tradition that left its mark on almost every corner of Ireland.

The fulacht fia as a monument type has attracted considerable archaeological debate. Some researchers have proposed alternative uses, including brewing, textile dyeing, or bathing, given that the basic technology of hot-stone boiling is versatile enough to serve any of these purposes. Whatever their function, the sites speak to a sustained and organised use of the landscape over many centuries. Mayo, with its boggy terrain and abundant watercourses, is particularly well supplied with them, and the Knockmore example sits within a county where prehistoric settlement has left a dense and varied record, from megalithic tombs to ring forts and beyond.