They appear as low, horseshoe-shaped mounds, typically found near water, and are thought to date mostly from the Bronze Age. The leading theory holds that they were cooking sites: a trough dug into the ground would be filled with water, and stones heated in a nearby fire were dropped in to bring the water to a boil. The burnt and shattered stones, discarded after use, gradually accumulated into the distinctive mounds that survive today. One such site sits at Ballynagranagh in County Clare, a quiet mark on the landscape that most people would walk past without a second glance.

The interpretation of these sites has shifted considerably over the decades. Some archaeologists have proposed that fulachtaí fia served purposes beyond cooking, including textile processing, leather working, or even bathing. Experimental archaeology has shown that the hot-stone method is a surprisingly efficient way to heat water, lending credibility to a range of domestic and industrial uses. The Clare landscape is particularly rich in prehistoric activity, and a fulacht fia at Ballynagranagh fits into a broader pattern of Bronze Age settlement and land use across the county, where low-lying ground near streams or marshy areas was clearly valued by early communities for sustained, repeated activity over generations.