That vivid colour is itself a clue, the levelled eastern portion of a fulacht fia, a type of prehistoric cooking site found across Ireland in considerable numbers, typically dating from the Bronze Age. The standard interpretation is that stones were heated in a fire and then dropped into a water-filled trough to bring it to the boil, leaving behind the characteristic blackened, fire-cracked debris that gives these mounds their distinctive appearance and composition.

The mound at Barnalyra is roughly subcircular, measuring approximately nine to ten metres across and surviving to a height of around 0.65 metres. It sits on the banks of a north-west-flowing stream, and the black soil packed with burnt stone is typical of the type. A drainage channel, about 1.2 metres wide, cuts through the western side of the mound on a north-west to south-east axis, and a ruined field wall of similar orientation has sliced across the eastern half, effectively flattening that section and exposing it to the air. Brambles have since crept across much of what remains. What makes the site particularly interesting in its landscape context is that it is not isolated. Three further fulacht fia lie along the same stream valley, the nearest less than half a metre to the north-north-west and the furthest around 55 metres away, suggesting repeated or sustained prehistoric activity concentrated along this single watercourse.