The surface is not large, roughly two and a half metres across, and it barely rises above the surrounding bog. But almost every centimetre of its usable face has been worked.

The decoration follows a clear internal logic. A large oval groove, about two metres wide and just over a metre deep, was incised into the rock to form an enclosing boundary, and most of the motifs sit within it. Rock art of this type typically involves cupmarks, which are small hemispherical hollows hammered or ground into stone, and cup-and-ring marks, where one or more concentric grooved rings surround a central cup. At Carhoomeengar, the range moves from simple cupmarks through cup-and-ring and cup-and-two-ring combinations up to a single cup-and-three-rings motif, thirty-five centimetres across, incorporated into the northwest side of the enclosing oval. That outermost motif has two parallel radial lines running from the inner ring out through to the edge, a detail that appears in Atlantic rock art more broadly but is not common. A groove runs east to west across the decorated area, meeting a natural crack in the rock as though the prehistoric carver had incorporated an existing feature into the design. On the southeast portion of the surface, six lightly incised circles sit apart from the main composition; one interpretation, offered by researcher Coyne, is that these represent preparatory outlines made before the deeper carving began, a kind of prehistoric draft that was never fully worked up.