It is a fulacht fia, a type of monument found in its thousands across Ireland, and one of the more quietly persistent puzzles in Irish archaeology. The basic form is always similar: a horseshoe or kidney-shaped mound built up from the debris of repeated fire-cracking, the process of heating stones in a fire and then plunging them into a water-filled trough to bring it to the boil. The mounds are the accumulated waste of that process, stones that had cracked and shattered beyond further use, piled up over what may have been many generations of use. What exactly was being cooked, or whether cooking was even the primary purpose, remains a matter of debate.

The Ballynaguilla example is a reasonable specimen of the type. The mound measures roughly 22.7 metres north to south and 22 metres east-northeast to west-southwest, with a surviving height of about 0.8 metres, though it has been partially levelled over time. Its opening, around 2.7 metres wide, faces west-southwest, which is typical of the form. The northern end of the mound has survived better than the southern end, though even there an oval depression, approximately 6 metres by 3 metres and about 0.4 metres deep, sits close to the northern edge. That hollow is likely the ghost of the original trough, the pit into which water was gathered and stones were dropped. The kidney shape of the mound wraps around that central working area, a physical record of where people stood and discarded spent material across what may have been centuries of Bronze Age activity.