Sitting in marshy ground on the northern side of a stream, it takes the form of an overgrown horseshoe-shaped mound of burnt and fire-cracked material, measuring roughly 22 metres long, 13 metres wide, and rising to about 1.4 metres in height. The opening, some 3.6 metres across, faces north.

A fulacht fia, sometimes called a burnt mound, is essentially the debris left behind by a Bronze Age cooking method. The basic principle involved heating stones in a fire and then dropping them into a water-filled trough, usually timber-lined, dug into the ground nearby. The stones crack and shatter with repeated heating and cooling, and it is the accumulation of these spent, blackened fragments that builds up over time into the characteristic horseshoe shape, with the trough sitting in the open centre. The method was efficient enough to be used repeatedly over generations, which is why so many of these mounds survive at all. The location at Beenreagh is typical of the type: close to a water source and in low-lying, naturally wet ground, both of which were practical requirements for keeping the trough filled. Most Irish examples date to the Bronze Age, broadly speaking somewhere between 1500 and 500 BC, though the precise period of use at any individual site is rarely easy to pin down without excavation.