These low, horseshoe-shaped mounds, typically found near water, are the remnants of ancient cooking or processing sites, built up over time from heat-shattered stone. The standard explanation is that they were used for boiling water, with stones heated in a fire and then dropped into a water-filled trough, though debate continues about whether the troughs served for cooking meat, brewing, bathing, or some combination of purposes. The example recorded near Cashel in County Mayo is one such site, sitting within a county that holds a considerable number of these monuments.

Fulachtaí fia date broadly from the Bronze Age, roughly 1500 to 500 BC, though some have produced earlier or later dates. The characteristic mound forms because the fire-cracked stones, once spent and useless for retaining heat, were simply discarded at the edges of the working area. Over centuries, these dumps of reddish, fragmented stone became the low crescent-shaped banks that archaeologists now recognise across boggy ground and river margins throughout Ireland. Mayo, with its abundance of wet lowland and upland terrain, is particularly well supplied with them, and many remain unexcavated, their precise character and date unconfirmed.