A fulacht fia, the term for these prehistoric burnt mound sites found in great numbers across Ireland, typically consisted of a mound of fire-cracked stone accumulated beside a water source, where stones heated in a fire were dropped into a water-filled trough to bring it to boiling point. At Barnacoyle Big, the site emerged only because a larger than usual strip of topsoil was removed to allow storage and parking during construction work for the Hollybrook to Wicklow Bord Gáis Éireann pipeline. Without that practical necessity, the spread of charcoal and shattered stone might have remained buried and unremarked.

Excavation under licence in 2001 opened an area of roughly 6.5 metres by 6 metres by hand, revealing a sequence of deposits tied to a stream that no longer follows its original course. The stream running east-west beside the nearby road was, according to the landowner, redirected into its current channel only in the 1960s, but the original loop of water was still legible in the landscape to the west of the site. Beneath river gravels and water-worn limestone boulders lay a waterlogged layer of grey sand with charcoal flecking, from which a sherd of Bronze Age coarse ware was recovered, placing human activity here somewhere in the broad span of the Bronze Age. Above this, at the point where the gravels met natural boulder clay, excavators found the characteristic spread of heat-shattered limestone and sandstone fragments, ranging from around 40mm to 100mm across, set in a compact black clay matrix roughly six metres long and up to 26 centimetres deep. No trough was found within the excavated area, which was only a portion of what is almost certainly a larger site; the evidence points to sustained activity along the bank of the former stream, with the exposed material representing just one fragment of a more extensive picture.