It is not much to look at from a distance, barely 0.6 metres high, but the dark fill and the shallow depression at its centre mark it out as something quite specific: a fulacht fia, one of the most widespread yet persistently mysterious monument types in the Irish landscape.

A fulacht fia is, in essence, the debris left behind by an ancient cooking or industrial site. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil. The stones, cracked and spent by repeated heating, were discarded in a horseshoe-shaped mound around the trough, which accounts for the characteristic dark, burnt-stone fill seen at sites like this one. The mound at Knockaunakill measures roughly 15 metres on its longer north-west to south-east axis and 12 metres across, and the shallow circular depression near its centre, about 3 metres in diameter, almost certainly marks where the trough once sat. The south-western edge of the mound is slightly indented, and the north-eastern side drops sharply down into the stream gully below, a fall of around 1.4 to 1.5 metres. The stream itself would have been the water source, conveniently close and bordering a wide expanse of bog that opens out to the north and east. Roughly 100 metres to the west lies a rath, a circular earthwork enclosure typically associated with early medieval settlement, suggesting this corner of Mayo held human activity across a long span of time, though whether the two monuments are directly related in date or use is not known.