That absence is itself a quiet curiosity. The mound measures roughly 12.4 metres north to south and 9 metres east to west, rising to a modest height of 0.9 metres, and its interior is composed of shattered sandstone fragments bound together in a dark, charcoal-rich soil. This is a fulacht fia, a type of prehistoric cooking site found widely across Ireland, typically beside water. The basic principle involves heating stones in a fire, then dropping them into a water-filled trough to bring it to the boil. The shattered, heat-cracked stones that result are piled to one side over repeated use, gradually forming exactly the kind of burnt mound visible here.

Fulachtaí fia are most commonly associated with the Bronze Age, though some have been dated to later periods. The characteristic kidney or horseshoe shape of these mounds is not accidental; it reflects the arrangement of the site, with the trough sitting in the open area and the discarded burnt stone accumulating around it in a curve. At Barnacoghil, that open area faces west. The stream running along the eastern edge of the boggy pasture would have supplied the water. The black, charcoal-saturated soil speaks to repeated, sustained use over time, though precisely when this particular site was active remains unknown.