They appear as low, horseshoe-shaped mounds, typically found near water, and are thought to represent ancient cooking or processing sites where stones were heated in fire and then dropped into a trough of water to bring it rapidly to the boil. The Lack townland example in County Mayo is one of countless such sites recorded across the island, a modest earthwork that has endured in the landscape for somewhere between three and four thousand years while the farmland around it has been worked, divided, and renamed many times over.

The mechanics of a fulacht fia are straightforward enough to reconstruct. Repeated heating and sudden cooling causes stones to crack and shatter, and the discarded fragments accumulate over time into the characteristic mound that survives today. What is less settled is the full range of activities these sites served. Cooking meat is the most widely cited explanation, but experimental archaeology has raised the possibility that the heated water was also used for bathing, textile processing, or even brewing. The site at Lack has not, on the basis of available information, been the subject of published excavation, so its particular history remains unexamined at close quarters.