Typically appearing as low, horseshoe-shaped mounds of burnt and fire-cracked stone, they are found beside streams and boggy ground, and the one recorded at Cunnagher in County Mayo is a quiet example of a type that appears in almost every county. The Irish term translates loosely as "cooking pit of the deer," and for much of the twentieth century that is exactly what archaeologists assumed they were: outdoor cooking sites where water was heated by dropping fire-heated stones into a trough, bringing it to a boil without direct flame. More recently, scholars have proposed alternative uses, including bathing, brewing, or textile processing, and the debate has not been fully settled.

Most fulachtaí fia date to the Bronze Age, roughly 1500 to 500 BC, though some have produced dates stretching into the Iron Age. The characteristic mound builds up over repeated use, as spent, shattered stone is raked away from the trough and piled to either side, forming the distinctive crescent shape that survives in the landscape long after the wooden trough, the fire, and the people who tended them have vanished entirely. Mayo has a particularly dense distribution of such sites, many preserved beneath peat that accumulated over millennia, which can both protect the archaeology and make individual sites difficult to locate or examine on the ground. The Cunnagher example sits within this broader pattern, a low earthwork in a county where the land itself has preserved a great deal that elsewhere was ploughed or built over.