It measures roughly twelve metres north to south and eight metres east to west, rising only about thirty centimetres above the surrounding ground. That modest rise conceals a mass of burnt and shattered stone, the characteristic signature of a fulacht fia.

A fulacht fia, sometimes called a burnt mound, represents one of the most common archaeological monument types found across Ireland. The working principle was straightforward: stones were heated in a fire, then dropped into a water-filled trough to bring the water to a boil, and the repeated thermal shock caused the stones to crack and fragment. Over time, the discarded burnt stone accumulated into a horseshoe-shaped or oval mound beside the trough. These sites are generally associated with the Bronze Age, though their precise function has been debated, with cooking the most widely accepted explanation and other uses, including hide processing or bathing, sometimes proposed. The presence of a stream immediately to the east of the Lettergorman mound fits the pattern well; a reliable water source was essential to the whole operation, and fulachta fiadh are almost invariably found close to running water or boggy ground.