It is made almost entirely of burnt and fire-cracked stone, the accumulated debris of repeated prehistoric cooking. Somebody, at some point after it fell out of use, tipped field clearance material across the top of it, which is how ancient sites often survive: not through careful preservation, but through casual burial.

The mound is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, particularly in low-lying or waterlogged ground near water sources. The typical method involved heating stones in a fire, then dropping them into a water-filled trough to bring the water to a boil. The stones shatter with repeated heating and cooling, and the discarded fragments build up over time into the distinctive horseshoe-shaped or circular mounds that still dot the Irish countryside in their thousands. What makes the Kilcullen example quietly striking is not the mound itself but its company. Three further fulachta fiadh lie immediately to the south-east, to the east, and roughly sixty metres to the north-east. Four of them, clustered within a short distance of the same stream, suggesting this particular stretch of ground saw repeated, perhaps sustained, use over time.