Twelve metres long and twelve metres wide, it is made almost entirely of burnt and fire-cracked stone, the accumulated debris of prehistoric cooking. This is a fulacht fia, one of the most common archaeological monument types in Ireland, and what makes this particular spot unusual is not the mound itself but what surrounds it: two more fulachta fiadh lie within 180 metres, one to the north-north-east and another further to the north-east, forming a loose cluster in the same stretch of ground.

A fulacht fia is essentially the remains of a Bronze Age cooking site. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to boiling point. The stones shatter with repeated heating and cooling, and over time the discarded fragments accumulate into the horseshoe-shaped or spread mounds that survive today, often beside streams or in low-lying damp ground. The siting of this example on the northern bank of a stream fits that pattern precisely. The presence of three such sites in close proximity at Ballynacaheragh is not without parallel in Ireland, where fulachta fiadh are frequently found in groups, suggesting repeated use of a favoured location over generations, though the relationship between individual sites and whether they were used simultaneously or sequentially is rarely easy to establish from surface evidence alone.