It is roughly circular, about eighteen metres across, and rises to just a metre at its highest point on the eastern side. Beneath the grass, however, the mound is composed almost entirely of heat-shattered stone fragments packed into charcoal-rich soil, the accumulated debris of repeated high-temperature activity carried out here long ago. A single large stone protrudes from the surface just north of centre, and a faint depression on the southern edge may mark where a trough once sat.

This is a fulacht fia, a type of prehistoric cooking or heating site found in enormous numbers across Ireland, most dating to the Bronze Age. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water rapidly to a boil. The stones shattered with thermal shock after repeated use and were discarded in a crescent-shaped mound around the trough, which is precisely what accumulated here beside the Fiddaunglass. What makes this particular site more than a solitary curiosity is its context: it belongs to a linear cluster of similar burnt mounds strung along the course of the same stream. The proximity of running water was not incidental; a reliable water source was a basic requirement for the whole process, and the Fiddaunglass appears to have drawn repeated use across what may have been a considerable stretch of time. The mound is sheltered to the north by rising ground, an old east-west field bank separates it from the stream four metres to the south, and a later field drain clips its eastern edge, all of which speak to the long, layered history of this small patch of Mayo pasture.