What lifts it from the ordinary is a small cup mark pressed into its surface, a deliberate hollow made by human hands at some point in prehistory, and the kind of detail that rewards those who look closely rather than broadly.

Cup marks are among the most widespread yet least understood forms of prehistoric rock art found across Ireland and Britain. They are simple in appearance, typically a shallow circular depression ground or pecked into stone, but their purpose remains genuinely uncertain. Ritual, territorial, astronomical, or something else entirely; no consensus has settled the question. The example at An Choill Mhór was identified as rock art by George Currie in 2018, a relatively recent recognition that adds this boulder to the broader, still-growing record of such markings documented across the Irish landscape.