What makes this one quietly notable is the combination of its modest but precise dimensions, roughly ten metres north to south and eight and a half metres east to west, rising only about eighty centimetres above the surrounding ground, and its apparent invisibility to the Ordnance Survey cartographers who mapped the area across multiple editions of the six-inch series. It simply was not recorded, and so it sat unacknowledged in the bog until a field survey caught up with it.

Fulachtaí fia, the plural form, are generally interpreted as Bronze Age cooking places, though theories about their function have expanded over the years to include brewing, hide-working, and bathing. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring it to a boil, a process that cracked and shattered the stones repeatedly. The discarded fragments accumulated in a characteristic horseshoe or kidney shape around the trough, which is exactly what survives at Barnacoghil. Here the mound is composed of shattered sandstone held in a matrix of dark, fire-blackened soil, and the open eastern side still has a large stone slab, about seventy centimetres long, protruding from the turf along its southern edge. That proximity to a stone circle, sitting only about seven metres to the south, adds an intriguing dimension, suggesting this corner of boggy Sligo was a place of repeated or varied activity during prehistory, though the relationship between the two monuments, if there is any direct relationship at all, remains unclear. Two smaller oval mounds lie just to the east and south-east of the fulacht fia, but these are thought to be upcast from a nearby drainage ditch rather than archaeological features in their own right.