They appear as low, horseshoe-shaped mounds, typically beside a stream or boggy ground, and are thought to date from the Bronze Age, roughly 1500 to 500 BC. One such site lies at Ballyline in County Clare, a quiet addition to a class of monument that continues to puzzle archaeologists despite its sheer abundance.

The standard interpretation of a fulacht fia holds that it functioned as an outdoor cooking site. The mound itself is the accumulated debris of fire-cracked stone, built up over repeated use. Nearby water was drawn into a trough, stones were heated in a fire and then dropped into the water to bring it to the boil, and meat was cooked within. Experiments have confirmed the method works efficiently. Some researchers, however, have proposed alternative uses, including textile processing, bathing, or brewing, and the debate has not been fully settled. The Clare landscape is particularly well populated with these monuments, with the county's mix of wetland margins and river corridors offering the kind of waterlogged ground that both suited their original use and helped preserve them through the millennia.