It measures roughly 25 metres east to west and 19 metres north to south, and in its north-eastern quadrant there is a shallow, roughly circular depression, about 4.4 metres across and 0.7 metres deep. The mound itself is composed of burnt material, the characteristic dark, crumbly residue that marks it out as a fulacht fia.

A fulacht fia is a type of prehistoric cooking site found in enormous numbers across Ireland, particularly from the Bronze Age. The basic principle involves heating stones in a fire, then dropping them into a water-filled trough to bring the water to boiling point. The stones crack and fragment with the repeated heating and cooling, and the discarded pieces accumulate over time into a horseshoe-shaped or oval mound of fire-shattered rock and charcoal. The depression at Knockduff likely represents the trough itself, the focal point around which the whole process turned. These sites are almost always found near a water source, and many thousands have been recorded across the island, yet each one represents repeated, deliberate activity by people who returned to the same spot, possibly over generations.