A fulacht fia, the term used for the crescent-shaped mounds of fire-cracked stone left behind by prehistoric cooking or heating activity, typically appears as a kidney or horseshoe of burnt debris beside a water source. At Knockduff, that relationship between mound and water was particularly direct: local accounts describe the horseshoe of burnt material as having actually surrounded the well, with its open end facing north, as though the structure had been deliberately oriented around the water rather than simply placed near it.

The mound itself was a substantial one before it was lost, measuring roughly fourteen metres east to west and sixteen metres north to south, rising to about ninety centimetres in height. That volume of scorched and shattered stone accumulates over time through a process archaeologists associate with repeatedly heating stones in a fire and then dropping them into a water-filled trough to bring it to the boil, a method used across Ireland during the Bronze Age for cooking, and possibly for other purposes such as bathing or textile processing. The well the mound encircled is now dry. In 1976, the mound was levelled, removing the visible archaeology from the pasture where it had survived for perhaps three thousand years or more.