It is made almost entirely of burnt and fire-cracked stone, the accumulated debris of repeated high-temperature use over what were likely centuries. The mound measures eighteen metres in length and fourteen metres wide, rising less than a metre above the surrounding ground, with a southeast-facing opening of just under four metres across. That opening is the clue to what this structure once was.

This is a fulacht fia, a type of site found in large numbers across Ireland, particularly in low-lying or waterlogged ground where a ready water supply was accessible. The name is an early Irish term associated broadly with cooking or heating, and the standard interpretation is that these sites functioned as outdoor cooking places, most likely during the Bronze Age, roughly 1500 to 500 BC. The method involved heating stones in a fire until they were extremely hot, then dropping them into a water-filled trough to bring the water rapidly to the boil. Repeated heating and quenching eventually caused the stones to fracture and become useless, and so they were discarded to the side, building up over time into exactly the kind of crescent-shaped mound visible at Caherbirrane. Some researchers have proposed alternative uses, including bathing, textile processing, or brewing, and the debate continues, though the burnt-stone mound itself remains one of the most recognisable archaeological forms in the Irish landscape.