The one at Ballynahallia in County Kerry is a quiet example of a type that appears so frequently in low-lying, waterlogged ground that it is easy to walk past one without registering what it is. The typical form is a horseshoe-shaped or kidney-shaped mound of shattered, fire-cracked stone, surrounding a sunken trough that was once lined with timber or stone. The working theory, broadly accepted though still debated, is that these were cooking sites. Stones were heated in a fire, dropped into a water-filled trough to bring it to the boil, and used to cook meat, possibly wrapped in straw or hides. Experiments have shown the method works efficiently, which has helped the cooking interpretation gain ground, though some researchers argue the sites may also have served as sweat houses or processing areas for textiles.

Most fulachtaí fia in Ireland date to the Bronze Age, roughly between 1500 and 500 BC, though some have produced dates from the Early Medieval period. The mounds themselves are the accumulated debris of repeated burning and quenching, the stone fracturing each time it was plunged into cold water and eventually discarded to form the distinctive raised crescent shape that survives in the landscape. They tend to cluster near streams or springs, the ready water supply being as essential to their function as the fire itself. Kerry has a notable concentration of these monuments, a reflection both of the county's Bronze Age activity and of the boggy, well-watered terrain that preserved the mounds over millennia. The specific setting of the Ballynahallia site, in the townland whose name derives from the Irish for something in the area of a water source or ford, fits the broader pattern neatly.