This is a fulacht fia, a type of prehistoric cooking site found in enormous numbers across Ireland, typically dating from the Bronze Age. The horseshoe shape is characteristic: a trough was dug, often lined with wood or stone, and filled with water. Stones were heated in a nearby fire and dropped into the trough until the water boiled, at which point meat could be cooked. The broken, heat-fractured stones were then discarded to one side, and over centuries of repeated use they accumulated into the distinctive mounded crescent that survives today.

The mound at Knockduff measures eight metres by seven metres and rises to about 0.6 metres at its highest point, though the southern side is noticeably lower. Its opening, roughly four metres wide, faces north-northeast. The site sits in marshy ground, which is entirely typical: fulachtaí fia are almost always found close to a reliable water source, whether a stream, a spring, or boggy terrain that holds water naturally. What makes the Knockduff example quietly interesting is that it does not stand alone. A second fulacht fia lies approximately 200 metres to the west-northwest, suggesting that this particular stretch of low-lying ground saw repeated or sustained prehistoric activity rather than a single episode of use.