The term, loosely translated from Old Irish, refers to ancient cooking or processing sites, typically identified today as a horseshoe-shaped mound of burnt and cracked stone beside a marshy hollow or stream. The one at Carrowneden in County Mayo is one such site, a low, dark signature in the landscape that has quietly outlasted almost everything built since.

The standard interpretation holds that fulachtaí fia were used by heating stones in a fire and dropping them into a water-filled trough, bringing the water to a boil rapidly enough to cook meat or process other materials. They cluster most densely in the Bronze Age, roughly 1500 to 500 BC, though some sites have produced dates ranging earlier or later. The burnt mounds that survive, composed largely of fire-shattered stone discarded after each use, can reach impressive dimensions after repeated episodes of activity over generations. Carrowneden sits in a part of Mayo with a dense prehistoric landscape, where bog and marginal land have preserved features that elsewhere were long ago ploughed or built over.