They appear as low, horseshoe-shaped mounds, typically found near water, and represent what archaeologists believe were Bronze Age cooking sites, though theories about their use have ranged from outdoor kitchens to brewing stations to communal bathing places. The example recorded at Ballynaraha in County Kilkenny is one of these quiet survivors, a crescent of earth and burnt stone that has endured in the landscape for perhaps three or four thousand years.

The basic mechanism of a fulacht fia was straightforward. A trough, often timber-lined, was filled with water, and stones heated in a nearby fire were dropped in to bring the water to a boil. The broken, heat-shattered stones accumulated over repeated use, gradually building up the characteristic mound that still marks these sites today. That burnt and fire-cracked stone, known as debitage, is often the most visible clue that a mound has prehistoric cooking origins rather than a more recent agricultural explanation. Ballynaraha sits in a part of south Kilkenny where the landscape has retained a good deal of its older character, and the presence of a fulacht fia there fits a pattern seen across river valleys and low-lying ground throughout Leinster.