To walk past it, you might take it for a natural rise in the ground. It is, in fact, the remains of a fulacht fia, one of the most numerous and least-understood monument types in the Irish archaeological landscape. These are the cooking sites, or possibly bathing or dyeing places, of prehistoric communities, typically dating from the Bronze Age. The standard interpretation involves heating stones in a fire, dropping them into a water-filled trough until the water boiled, and using that heat for cooking. The shattered stones, cracked by the repeated shock of heating and cooling, were then discarded to one side, building up over time into the distinctive burnt mound that survives today.

Fulachta fia are found in their thousands across Ireland, almost always in low-lying, wet ground near a stream or natural water source, and Mayo has a considerable concentration of them. The specific site at Lack has been recorded as a protected monument, quietly sitting within the wider pattern of prehistoric activity that marks this part of Connacht. The name fulacht fia is sometimes translated as cooking place of the deer or cooking pit of the Fianna, linking it in folklore to the roaming warrior bands of Irish mythology, though the archaeological reality is more prosaic and more interesting: these were functioning, practical installations used repeatedly over generations, accumulating their mounds stone by stone across what may have been centuries of use.