What was once a substantial mound, oblong in shape and roughly 35 feet from north to south and 25 feet from east to west, rising to a height of around six feet at its centre before tailing off to a few inches at the edges, has since flattened into agricultural ground with no visible surface trace remaining. The locals who knew it in earlier generations had their own interpretation: they regarded it as an old pagan burial ground. The archaeology told a different story.

When P. J. Hartnett examined and described the site in 1939, he found beneath the turf-covered exterior a fill of black humified earth mixed with brittle burned stone, the characteristic signature of a fulacht fia. These are ancient cooking sites, typically Bronze Age in origin, where stones were heated in fire and then dropped into a water-filled trough to bring it to the boil. The cracked and shattered stones that resulted from repeated heating and sudden cooling were piled up over time into the distinctive mounds that survive across the Irish landscape, often in low-lying or waterside ground. The blackened, fire-fractured stone at Oldcastle matched this pattern precisely, and the mound's imposing dimensions suggest it was used repeatedly over a considerable period. The misidentification as a burial ground is not unusual; fulachta fia are frequently earthwork-like in appearance, and without excavation their origins remain opaque to the untrained eye.