This is a fulacht fia, a type of ancient outdoor cooking site found in great numbers across Ireland, typically consisting of a horseshoe-shaped mound of fire-cracked stone accumulated beside a water source. The stones were heated in a fire, then dropped into a water-filled trough to bring it rapidly to the boil. The mound itself is the discard heap, built up over repeated use as spent, fractured stone was shovelled aside. At Rathmacostello, the elongated mound measures roughly thirteen metres along its longer axis and just under half a metre in height, and the curved western face of it encloses a slight hollow that may mark where the original trough once sat.

The site sits in sheltered, damp pasture on the western side of a spring that rises at the foot of a slope and flows northward into the River Deel just fifteen metres away. That proximity to water is entirely typical; fulachtaí fia are almost always found close to springs, streams, or boggy ground, and the damp, low-lying character of this spot would have made it well suited to the purpose. What makes the Rathmacostello location particularly striking is the density of prehistoric activity concentrated here. A second fulacht fia lies only seven metres to the east, on the opposite bank of the same spring, and a burnt mound of related character sits fifteen metres to the north-west. A rath, a circular earthen enclosure of the early medieval period, occupies higher ground just fifteen metres to the south. The eastern edge of the fulacht mound has been absorbed into a field bank running on a north-south axis, and past disturbance has left its original shape and extent somewhat uncertain.

The mound is on agricultural land, and the landscape here is quiet and largely unremarkable at first glance, which is precisely what makes the concentration of sites so unexpected. Visitors with an eye for subtle earthworks should look for the low, heather-flecked rise in the pasture and the faint hollow within its curve, set against the line of the spring feeding down toward the Deel.