Nothing about it announces itself as remarkable. Yet the semi-circular recessions cut into the rock, and the wide scree slope of chert fragments spilling down through the oak and hazel woodland below, may represent one of the earliest industrial sites in Ireland, a place where Mesolithic people, some seven or more thousand years ago, came specifically to extract raw material for making tools.

Chert, a flint-like stone formed within limestone deposits, was the flint of inland Ireland, knapped and shaped into blades, scrapers, and points throughout the Mesolithic period. The particular variety found here is known as festooned chert, distinguishable from the more common black glassy type by its characteristic thin dark lines looping and ringing through dark brown stone. In 2006, Aimee Little and Dr Matthew Parkes of the Natural History Museum of Ireland visited the site and examined the scree at the base of the outcrop, finding several pieces that showed clear signs of secondary working, meaning they had been shaped by human hands rather than simply fractured by natural forces. Pick-like forms were identified among the debris. Dr Parkes also noted areas on the outcrop face where the pattern of bed recession could not be explained by natural weathering alone, pointing instead to deliberate quarrying activity. The connection does not stop at the hill itself: festooned chert tools recovered from Clonava Island, approximately 8.6 kilometres to the north-west, are thought to have originated here at Knockeyon, suggesting the material was transported across the landscape. A late Mesolithic wetland site at Corralanna, 12.5 kilometres to the north-north-west, adds further context to what appears to have been a network of Mesolithic activity across this part of the Irish midlands.

The quarry face sits just 75 metres south of the hill's summit, with the shoreline of Lough Derravaragh only 250 metres to the south-west and a church dedicated to St Eyon a little closer than that. The scree slope running down into the woodland below the outcrop is where the worked chert fragments were found, and the festooned chert itself, with its looping dark lines, is visually distinctive enough to be recognisable once you know what to look for.