These mounds, found in their thousands across Ireland, are the by-product of an ancient cooking or industrial process in which stones were repeatedly heated in fire and plunged into a water-filled trough, causing them to crack and blacken over time. The discarded, heat-shattered stone gradually accumulated into the horseshoe or oval shapes that survive today, often in damp, low-lying ground where a natural water source was close at hand.

The mound at Coolacullig sits in pasture within a natural hollow, a setting entirely typical of the type, since fulachtaí fia are almost always found in wet or seasonally waterlogged areas. It is a substantial example: roughly 26 metres along its north-south axis, 14 metres across, and rising to about 1.2 metres in height, giving it a quiet but unmistakable presence in the field. A drain cuts through its northern side, and a further spread of burnt material, some 8 metres long, extends north from the drain at its eastern end, suggesting the mound's original footprint was once somewhat larger, or that activity in this area was not confined to a single episode. Most fulachtaí fia are thought to date to the Bronze Age, broadly speaking the period from around 2000 to 500 BC, though some sites have produced earlier or later dates.