They appear as low, horseshoe-shaped mounds, typically found close to water, and represent what was essentially a Bronze Age cooking technology. The method involved heating stones in a fire and dropping them into a water-filled trough until the water boiled, a process that cracks and blackens the stones over time. The mound itself is the accumulated debris of those spent, shattered stones, built up over repeated use across what may have been generations. The example at Levallinree in County Mayo is one such site, quietly occupying its place in the boggy terrain that fulachtaí fia seem almost to seek out.

The broader distribution of these monuments across Ireland tells a story of organised, repeated activity during the Bronze Age, roughly between 1500 and 500 BC, though some sites have returned earlier dates. Mayo has a considerable concentration of them, as the wet, marginal land that was less attractive for later intensive farming has preserved what elsewhere was ploughed away or built over. What fulachtaí fia were actually used for remains a matter of some debate among archaeologists. Cooking is the long-standing interpretation, and experiments have confirmed it works well, but bathing, textile processing, and brewing have all been proposed as alternative or additional functions. The honest answer is that they were probably put to several uses by the communities that built and returned to them.