It measures roughly nine metres across and rises only half a metre at its highest point, sloping gently down toward a westward-flowing stream. Grass and heather cover it now, but beneath the surface the material tells a different story: thousands of fire-cracked fragments packed into a matrix of black, charcoal-rich soil. This is a fulacht fia, and this particular one at Barnacahoge fits the type almost exactly as textbook examples describe it.

Fulachtaí fia are among the most common archaeological monuments in Ireland, found in their thousands, typically in low-lying or waterlogged ground near a water source. The general interpretation is that they were cooking sites, used during the Bronze Age, where water in a trough was heated by dropping fire-heated stones into it. The stones, repeatedly superheated and then plunged into cold water, crack and fracture and become useless, and so they are tossed aside after use, gradually building up the characteristic horseshoe-shaped mound that survives today. At Barnacahoge the proximity to the stream to the north would have provided a ready water supply, and the sheltered position at the valley's blind end, overshadowed on three sides by rising ground, would have offered some natural enclosure. A number of flat slabs lie scattered on the bog surface near the mound, though their precise function is unclear. Further up the slope to the south stands a cashel, a dry-stone walled enclosure typically associated with early medieval settlement, suggesting the area saw use across more than one period.