It measures roughly seven and a half metres across and rises only about forty centimetres from the surrounding ground. To a passing eye it might read as nothing more than a slight swelling in the field. In fact, it is a fulacht fia, one of the most common yet persistently mysterious monument types in the Irish archaeological landscape.

Fulachtaí fia (the plural form) are the remains of ancient cooking or processing sites, typically Bronze Age in date, though some extend into the early medieval period. The mound itself is the waste heap, built up over time from fire-cracked stone discarded after repeated use. The usual method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil. The location beside a stream at Castlederry fits the pattern almost exactly; proximity to a reliable water source was essential to the whole operation, and the majority of these sites are found in low-lying, wet ground close to running water. What precisely was being cooked, or whether cooking was even the primary function, remains genuinely debated. Proposals over the years have ranged from meat preparation to textile processing to sweat-house bathing, and no single explanation has settled the argument. The burnt and shattered stone that forms this particular mound, covered now in a skin of grass, represents many separate episodes of heating, use, and clearance, each one adding slightly to the profile of the heap.