The mound measures roughly 13 metres north to south and 11 metres east to west, rising only about 45 centimetres above the surrounding ground. That modest height is part of what makes these sites so easy to overlook, and so frequently ploughed away or built over before anyone notices what they are.

The mound is a fulacht fia, a type of site found in enormous numbers across Ireland, with County Cork alone holding thousands of recorded examples. The term, loosely translated from the Irish, is sometimes rendered as "cooking place of the deer," though the precise meaning has long been debated. The basic principle is well understood: a trough, often timber-lined or stone-lined and dug into the ground near a water source, was filled with water, and stones were heated in a nearby fire and dropped in to bring the water to a boil. The cracked, heat-shattered stones were then discarded in a pile beside the trough, and it is precisely this mound of burnt and broken material that survives at Ballysallagh and at sites like it across the country. Most fulachta fia date to the Bronze Age, roughly 2000 to 500 BC, though some were in use earlier or later. Their exact purpose remains a subject of some discussion among archaeologists; cooking is the traditional explanation, but brewing, hide-working, and bathing have all been proposed. What is certain is that the process was repeated many times over, since the mounds can represent centuries of accumulated use.