This is a fulacht fia, a type of prehistoric cooking site found across Ireland, typically consisting of a horseshoe-shaped mound of fire-cracked stone built up around a water-filled trough. The stones were heated in a fire and dropped into the trough to boil water, and over repeated use the shattered, blackened fragments accumulated into the characteristic mound. At Oatfield, that mound stretches roughly 13.5 metres east to west and rises to about 1.2 metres at its highest point before stepping down in two distinct platforms. Some of the blackened, friable stone remains visible, and a possible trough area can be made out beneath the vegetation at the eastern end, though the central portion is now entirely obscured by growth.

The site sits in decidedly boggy ground, which is typical for fulachta fiadh; the waterlogged conditions that make such places awkward to visit are often precisely what preserved them. It was first reported here by Micheál Mac Gearailt, and a separate record by the Discovery Programme had placed a fulacht fia at a nearby grid reference where no monument was actually found on the ground, suggesting both entries refer to this same mound at the north-eastern edge of the lake. The stone is described as sandstone, firecracked from repeated heating. Coolmeen Lake itself has a longer and darker association in the historical record. The 14th-century Irish chronicle Caithréim Thoirdealbhaigh, a narrative of the wars of the O'Brien dynasty in Thomond, records that a man named Lochlainn McNamara was beheaded at Coolmeen and that his head and body were buried in separate locations. Where those locations were, the chronicle does not say.