They appear as low, horseshoe-shaped mounds, typically beside a stream or marshy ground, and are thought to date from the Bronze Age, roughly 1500 to 500 BC. The prevailing theory holds that they were outdoor cooking sites: a trough dug into the earth and lined with wood or stone would be filled with water, then heated by dropping fire-cracked rocks into it until the water boiled. The burnt and shattered stones, discarded after each use, gradually accumulated into the characteristic mound that survives today. The one at Capparanny, in County Mayo, is one such site, quietly occupying its place in the landscape as it has for perhaps three millennia.

The precise details of this particular monument, its dimensions, condition, and any associated finds or features, are not currently available in the public record. What can be said is that Mayo has a notable concentration of fulachtaí fia, partly because the county's wet, boggy terrain was well suited to their use and has since helped preserve them. The sheer number of these sites across Ireland has led some researchers to question whether cooking was their only purpose; brewing, hide-working, and bathing have all been proposed as alternative or additional functions, and experimental archaeology has shown that all of these are technically feasible using the heated-trough method.