It measures about 8.6 metres east to west and 8 metres north to south, rising only a quarter of a metre above the surrounding ground. Unassuming as it looks, the mound is composed entirely of burnt material, the characteristic signature of a fulacht fia.

A fulacht fia is a type of prehistoric cooking or processing site found in considerable numbers across Ireland, typically Bronze Age in origin, though some examples span a wider period. The standard interpretation is that stones were heated in a fire and then dropped into a water-filled trough to bring the water to boiling point, the cracked and shattered stones being raked aside into a mound after use. Over time, that discard heap is exactly what remains: a low, horseshoe-shaped or circular rise of fire-cracked stone and charcoal-darkened earth. The one at Reanasup follows this pattern precisely. Its north-facing position on a slope may simply reflect where a reliable water source was once accessible, since proximity to a spring or stream was a practical requirement for sites of this kind.