The one recorded at Knockbaun in County Mayo is a quiet example of a type that archaeologists are still, in many respects, arguing about. The term itself, loosely translated from Old Irish, refers to a cooking place of the deer or wild animals, though that interpretation has been contested. What is not in dispute is the basic form: a mound of burnt and fire-cracked stone, typically horseshoe-shaped, surrounding a sunken trough that would once have been lined with wood or clay. The standard theory holds that stones were heated in a fire, then dropped into water-filled troughs to bring them rapidly to the boil, presumably for cooking, though brewing, hide-working, and bathing have all been proposed as alternatives.

Most fulachtaí fia date to the Bronze Age, roughly 1500 to 500 BC, though some have been found with earlier or later activity. They tend to appear near water sources, streams or boggy ground, which would have supplied the trough. The burnt mounds that survive today are essentially the discarded stone, cracked and blackened by repeated heating and cooling, heaped up over generations of use. Ireland has over four thousand recorded examples, yet individual sites like Knockbaun often remain underdocumented, known mainly as a mark on a map rather than a place with an excavated story attached to it.