This is a fulacht fia, a type of site found in great numbers across Ireland, typically interpreted as a Bronze Age cooking place. The basic principle involves heating stones in a fire, dropping them into a water-filled trough to bring it to the boil, and using that water to cook meat. The stones, cracked and shattered by repeated heating and rapid cooling, were discarded around the trough, and it is precisely those accumulations of fire-fractured stone, bound together in a dark, charcoal-rich matrix, that now form the visible mound.

The mound at Aderg measures roughly 13 metres north to south and 8.3 metres east to west, rising to about half a metre at its highest point on the northern edge. Its kidney shape is characteristic of the type, and the pronounced depression running east to west across its surface is likely where the trough once sat, with the deepest point towards the west suggesting that is where water was held and heated. The ground around it suits the site well; land that falls away to the north towards a drain or stream would have provided a reliable water source, which any such cooking place would have required. Adding a further layer of interest, faint linear depressions crossing the top of the mound on a northeast to southwest axis may represent old cultivation ridges, evidence that the mound was at some point pressed into agricultural use long after its original purpose had been forgotten. A second fulacht fia lies approximately 200 metres to the east, suggesting this corner of Mayo saw repeated or sustained activity during the Bronze Age.