What lies beneath, or rather what has accumulated over centuries, is the residue of a fulacht fia: a type of prehistoric cooking site found in enormous numbers across Ireland, typically identified by a horseshoe-shaped mound of fire-cracked stone and charred material. The method involved heating stones in a fire and dropping them into a water-filled trough to bring it to a boil, after which the spent, shattered stones were discarded to the side, building up the characteristic mound over repeated use. At Knockduff, that mound is the sole surviving indicator of what was once a place of regular, practical activity.

When a recorder named Broker visited in 1937, the mound was described as round, roughly ten feet across and four feet high. That description, modest as it is, tells its own quiet story: in the intervening decades the feature has diminished to something barely perceptible at ground level, reduced by the pressures of agricultural land use and time. Fulachtaí fia are generally dated to the Bronze Age, though some sites in Ireland have yielded evidence of use across much longer periods. Their persistent association with water sources, such as the well immediately to the north here, is consistent with the practical demands of the cooking process, which required a reliable and nearby supply.