This is a fulacht fia, a type of site found in considerable numbers across Ireland, consisting essentially of a horseshoe-shaped or mound-shaped accumulation of fire-cracked stones. The working theory, supported by experimental archaeology, is that these were outdoor cooking sites, where stones were heated in a fire and dropped into a water-filled trough to bring it rapidly to a boil. The mound itself is the discarded debris, stones shattered by repeated heating and cooling, piled up over many uses across what may have been centuries of intermittent activity.

The Knockduff example measures roughly 20 metres by 18 metres, standing just 0.45 metres high today, a considerably diminished presence compared with what was recorded by Broker in 1937, when the mound was described as approximately 30 feet in diameter and four or five feet high, and already noted as mostly removed even then. That progressive loss of height and material over the intervening decades is not unusual; fulachta fia were often quarried for hardcore or simply disturbed by agricultural activity without anyone quite registering what they were. What remains here is enough to identify the site, though little of the original volume survives. Notably, a second fulacht fia lies approximately 40 metres to the north, suggesting this stretch of ground beside the stream saw repeated or concurrent use, perhaps because the reliable water source made it a practical and repeatedly returned-to location.