It measures fourteen metres long, just over five metres wide, and rises about a metre from the marsh, with a shallow depression at its centre half a metre deep. This is a fulacht fia, one of the most common prehistoric monument types found across Ireland, and in some ways one of the most quietly puzzling.

A fulacht fia, sometimes called a burnt mound, is the accumulated debris of repeated high-temperature cooking, or possibly craft activity such as leather-working or textile production. The typical method involved heating stones in a fire, dropping them into a water-filled trough to bring it rapidly to the boil, and then discarding the spent, shattered stone nearby. Over generations of use, that discarded material built up into the horseshoe or oval-shaped mounds that survive today, often in their thousands across the Irish landscape. The central depression at Knocknagappul likely marks the position of the original trough, whether timber-lined or simply cut into the earth. The site's location beside a stream is entirely characteristic: a reliable water source was the one non-negotiable requirement. Most fulachtaí fia date to the Bronze Age, broadly between 1500 and 500 BC, though some are earlier or later, and their precise social function remains a matter of genuine archaeological debate.