Beneath the turf lies a spread of burnt material, the characteristic signature of a fulacht fia, and the same scorched debris reappears in a drain along the southern edge of the site, where erosion or drainage work has cut through the deposit and exposed it.

Fulachtaí fia are among the most common prehistoric monuments in Ireland, typically Bronze Age cooking sites where stones were heated in a fire and dropped into a water-filled trough to bring it to the boil. The resulting shattered, fire-cracked stone accumulated in a horseshoe or spread around the trough, forming the low mounds that survive today. The Knockduff example was recorded in 1937 by Broker, who described it as a semi-circle roughly fifteen feet in diameter and about five feet high, a reasonably substantial monument for the type. Notably, a second fulacht fia lies approximately forty metres to the south, suggesting this particular corner of north Cork saw repeated or prolonged use, perhaps because of a reliable water source that no longer reads obviously in the modern landscape.