This is a fulacht fia, a type of ancient cooking or heating site found in enormous numbers across the Irish countryside, and the example at Knockduff preserves the classic signature of the type: a crescent of burnt and fire-cracked stone, built up over repeated use into a mound that has survived long after everything else about the people who made it has been forgotten.

The Knockduff mound measures roughly 13 metres in length and nearly 10 metres across, rising to about three-quarters of a metre at its highest point. Its opening, some 3.2 metres wide, faces toward the east-north-east. That opening would originally have faced a trough, usually a timber-lined pit sunk into the ground and filled with water, which was heated by dropping fire-cracked stones into it. The stones, once too shattered to retain heat efficiently, were discarded to the sides, gradually building the characteristic horseshoe shape over many episodes of use. Fulachtaí fia are predominantly Bronze Age in date, though the tradition appears to have persisted across a considerable span of time, and they are found in their thousands across Ireland, typically in low-lying or marshy ground close to a water source.