The decorated surface of a large east-facing rock outcrop extends under the front elevation of the structure, meaning that at least one of its carved features, a curvilinear motif situated roughly twenty centimetres from its neighbours, simply disappears into the building's foundations. It is an oddly apt situation for prehistoric rock art, which has a habit of existing half in and half out of human comprehension.

The outcrop itself is a rough, fractured expanse of stone oriented northeast to southwest, its surface marked by natural striations, solution hollows, and fissures, all softened by a light covering of moss. Against this busy natural texture, the carved elements are deliberate and considered. The most prominent is a large heptagonal motif, roughly 54 centimetres by 42 centimetres, defined by a curvilinear groove about one and a half centimetres wide and four millimetres deep. Adjoining it to the north is a sub-rectangular feature, shallower and slightly less wide in its groove, measuring 35 centimetres east to west and 18 centimetres north to south. Rock art of this kind, geometric and abstract forms pecked or incised into exposed stone, is generally assigned to the Neolithic or Bronze Age periods, though precise dating remains difficult across the tradition as a whole. The site at Glannakilleenagh was first identified as rock art by George Dose on 21 March 1994, which means it entered the record relatively recently, well within living memory, despite being ancient in origin.

The decorated surface faces east and is described as relatively steep, which means the carved motifs would catch low raking light in the morning, the kind of light that makes shallow grooves legible where direct overhead sun would flatten them entirely. The pathway to the outbuilding's front door runs adjacent to the north of the outcrop, so the stone is not remote or difficult to approach in terms of its immediate surroundings, though it sits within a private house plot and access would depend entirely on the goodwill of whoever lives there.