Nobody thought it unusual enough to disturb. What the builders of that fence had incorporated into their rubble and mortar was a piece of prehistoric rock art, carved with a system of concentric circles and cup-marks whose meaning has not been recovered in the millennia since they were made.

Cup-and-ring marks are among the most widely distributed forms of prehistoric art in Atlantic Europe, consisting of shallow circular depressions, called cups, sometimes surrounded by one or more incised rings. The Knocknakilton stone is a relatively small slab, measuring 1.2 metres by 0.8 metres and no more than 15 centimetres thick, but its surface carries a notably detailed composition. The dominant motif is a large cup enclosed by three concentric circles, with a radial line cutting outward from the central cup through a gap in the innermost circle and continuing across the outer rings. A second radial line runs in the diametrically opposite direction, though this one was not carved; it follows a natural fissure in the rock, either incorporated deliberately by whoever made the design or a fortunate coincidence that later observers have read as intentional. Several further cup-marks are scattered across the surface, along with a cup-and-circle that may once have been complete before the stone began to spall. A companion stone, found close by on the same hillside, remains at the original location. The carved slab itself was documented by J. Cuppage in the 1986 Dingle Peninsula archaeological survey and has since been removed from the field fence for safekeeping.

The stone is now held by Oidhreacht Chorca Dhuibhne and displayed in the West Kerry Museum in Ballyferriter, where it can be seen in its rescued state, separated from both the hillside view it once overlooked and the fence it spent an unknown number of centuries quietly reinforcing.