What it actually represents is the accumulated debris of prehistoric cooking, a fulacht fia, one of thousands of such sites scattered across Ireland and most densely concentrated in low-lying, waterlogged terrain exactly like this.

A fulacht fia is, in essence, the waste heap left behind by a Bronze Age hot-stone cooking method. Stones were heated in a fire, dropped into a water-filled trough, and used to bring the water to a boil. Once cracked and spent, the stones were discarded to one side, building up over repeated use into the characteristic horseshoe or oblong mound that survives today. At Lisduff, that mound measures roughly 14 metres east to west and about 8 metres north to south, rising to just over a metre at its western end. It is composed of heat-shattered stone packed into a matrix of charcoal-rich soil, the direct physical signature of fire and repeated thermal shock. On the south-south-west side of the mound, facing the stream, there is a hollow roughly 2 metres across, marked by damp ground and rank grass. This is likely the position of the original trough, the point where water was collected and heated, and its orientation toward the stream makes practical sense. The whole mound is now capped with a layer of peaty soil and scrubby heather, which is typical of sites that have lain undisturbed in wet ground for millennia.

The location follows a pattern seen at fulachtaí fia across the country: close to a reliable water source, on reasonably flat ground, with slightly rising terrain nearby. The stream bend at Lisduff would have provided a natural collecting point for water, and the north bank position keeps the mound just clear of the worst of the seasonal flooding. Whether these sites served purely as cooking places, or also had other functions such as bathing or textile processing, remains debated among archaeologists, but the physical evidence at Lisduff, the burnt stone, the probable trough, the charcoal soil, is entirely consistent with the classic picture.