These low, horseshoe-shaped mounds, typically found near water sources, are the accumulated debris of a cooking method that involved heating stones in a fire and dropping them into a water-filled trough until the water boiled. The scorched and shattered stones were then raked aside, and over generations of repeated use the discarded material built up into the characteristic mound that survives today. The example at Cunnagher in County Mayo is one such monument, quietly occupying its place in the landscape without any great fanfare.

The fulacht fia tradition spans a long period of Irish prehistory, with most excavated examples dating to the Bronze Age, roughly 2000 to 500 BC, though some have returned earlier or later dates. Their function has been debated for decades. Cooking is the most widely accepted explanation, but experimental archaeology has shown that the same stone-boiling method works equally well for textile processing or bathing, and some researchers have argued for more varied uses. What they consistently share is proximity to water, whether a stream, a spring, or a boggy hollow, and the presence of that distinctive burnt mound, blackened with charcoal and dense with fractured, fire-cracked stone. Mayo has a particularly high concentration of such sites, partly a reflection of the county's wet and peaty terrain, which both preserved the organic material within the mounds and, perhaps, made reliable access to water a practical priority for prehistoric communities working the land.