A slice through the soil, exposed in a drainage cut on the south-western edge, reveals the deposit sitting nearly a metre deep, dark and compact, the accumulated residue of repeated firings long before the land was ever farmed in any recognisable modern sense.

A fulacht fia, sometimes called a burnt mound, is among the most common prehistoric monument types found across Ireland. The typical method involved heating stones in a fire, then dropping them into a trough of water to bring it to a boil, most likely for cooking meat. The shattered, fire-cracked stones were discarded into a mound nearby, which is what survives. What makes Coolnahane unusual is not this single site in isolation, but its context. Approximately 120 metres to the north-west lies a cluster of seven further fulachta fiadh, all within close proximity of one another. Whether this grouping reflects repeated use of a particularly favourable location, perhaps near a reliable water source, or represents some more organised and sustained activity, is not recorded, but the density of monuments in such a small area is striking by any measure.