The stone measures roughly two metres by one and a half, and across its sloping face someone, at some point in prehistory, carved circles within circles, rings around shallow cups, and dozens of plain cupmarks pressed into the rock. Cup-and-ring marks, as they are known, are among the most widespread yet least understood forms of prehistoric art in Atlantic Europe: concentric arcs and rings incised around a central depression, their purpose debated for generations without resolution. What makes the Derrynablaha boulder notable even within this tradition is the density and variety of its decoration, which covers three distinct sections of the surface and includes a cup-and-four rings motif with a diameter of 0.34 metres, one of the more elaborate examples of the form.

The boulder was documented in the early 1960s by the archaeologist Emmanuel Anati, who recorded it after it was deturfed, meaning the accumulated peat and soil that had been slowly swallowing it was cleared away to expose the full surface. That surface divides into a roughly rectangular central section, a smaller rectangular section to the north-west, and a triangular section to the south-east. The central section carries the most complex work: a cup-and-two rings at the northern end, the cup-and-four rings at the south, eight clearly defined cup-and-rings, and around twenty-nine individual cupmarks. The northern section adds another twenty or so cupmarks, the largest and deepest clustered at its far end, along with a line of picking and a series of five hollows running north-north-east to south-south-west. Natural cracks cross parts of the surface and in places the carving seems to negotiate them, with motifs placed both above and below a significant crack running through the eastern section. Whether that relationship was deliberate is one of the many things the stone does not give away.