Across its north-western face, three grooves run in parallel, roughly north-east to south-west, each V-shaped in cross-section. Near their south-western ends sit a possible cupmark and two solution pits. Cupmarks are shallow, roughly circular depressions pecked into rock surfaces, and they appear on prehistoric stones across Ireland and Britain, though their precise purpose remains genuinely uncertain. The solution pits here are natural hollows formed by chemical weathering of the rock, and their proximity to the carved grooves raises the question of whether the human-made and the natural features were ever understood, by those who used the stone, as distinct from one another.

The archaeologist M. J. O'Kelly, writing in 1958, proposed a decidedly practical explanation for the grooves: that they were used to sharpen pointed implements, the repeated drawing of bone, antler, or stone tools across the rock surface gradually cutting those characteristic V-shaped channels. It is a persuasive reading. Grooves of this kind are known from other Irish sites and are sometimes called grinding or polishing stones, used during the Neolithic or Bronze Age when ground-stone technology was central to daily life. The Coomasaharn boulder would, on this interpretation, be less a ritual monument than a kind of communal workbench, worn by generations of use into the landscape of the peninsula.