It measures over forty-six metres east to west and nearly eighteen metres north to south, rising about a metre above the surrounding ground. Where the stream has cut into its bank, burnt material is visible in cross-section, the telltale signature of a fulacht fia.

A fulacht fia is a prehistoric cooking site, typically Bronze Age in date, built around the repeated heating of stones in fire and then dropping them into a water-filled trough to bring it to the boil. The broken, heat-shattered stones accumulate over time into a horseshoe-shaped mound, which is what survives here at Knocknagappul. What makes this particular site notable is not just its size but its company: it is one of a group of four such monuments clustered along the same watercourse. These kinds of groupings are not unusual in Irish archaeology, since fulachta fiadh tend to favour stream banks where water was reliably accessible, but four in close proximity along a single stream speaks to sustained, repeated use of this corridor over what may have been a considerable period. The burnt material still exposed in the stream bank provides a direct, unmediated glimpse of the activity that created the mound, the charred residue of ancient hearthwork slowly re-emerging as the water continues to erode the edge.