They appear as low, horseshoe-shaped mounds, typically found near water, and are generally dated to the Bronze Age, roughly 1500 to 500 BC. The name, loosely translated from Irish, is sometimes rendered as "cooking places of the Fianna", though the link to any heroic tradition is almost certainly a later folk explanation. What archaeologists have established is the basic method: stones were heated in a fire, then dropped into a water-filled trough, bringing the water to a boil. The resulting mound is the accumulation of those fire-cracked, spent stones, discarded after each use.

The example recorded at Catstown in County Kilkenny is one of many such sites preserved, at least in name, across the townlands of Leinster. Kilkenny itself sits in a region where Bronze Age activity has left a considerable imprint on the landscape, and fulachta fia tend to cluster near streams and boggy ground, precisely the kind of terrain that made them practical for their original users. What exactly was being cooked, or whether cooking was even the primary function, remains genuinely open. Some researchers have proposed uses ranging from textile processing to bathing, and the debate has not been settled. The site at Catstown is a reminder that the ordinary rhythms of prehistoric life, the heating of water, the gathering of stone, the repeated use of a particular patch of ground, can leave marks that outlast almost everything built since.