These are cupmarks, one of the most widespread and least understood forms of prehistoric rock art found across Ireland and Britain: small, bowl-shaped depressions ground or pecked into stone, typically a few centimetres across and a centimetre or two deep. The Threewells slab has at least eight confirmed examples, each roughly 4.5 centimetres in diameter and 2 centimetres deep, along with a further five possible cupmarks. They cluster towards one end of the decorated face, near the field boundary, but show no obvious geometric arrangement or pattern.

The slab itself is sub-rectangular, measuring 1.14 metres long by 0.82 metres wide and 0.23 metres high, and its surface has taken on a mottled colouring over time, most likely the result of lichen growth on the granite. Dr. Clíodhna Ní Lionáin of the UCD Wicklow Rock Art Project documented the stone as part of a systematic effort by the School of Archaeology at University College Dublin to record and model prehistoric rock art across the county. The project produced a photogrammetric three-dimensional model of the slab, a technique that uses overlapping photographs to construct a precise digital surface, capturing the subtle topography of the cupmarks in a way that ordinary photography cannot. The slab sits around 300 metres northeast and uphill of another recorded rock art site in the same area, suggesting this part of Wicklow may once have held a concentration of marked stones, though what drew people to carve here, and when exactly they did so, remains open.