Get closer, and the surface of its south-south-westerly face begins to reveal itself: at least seven small circular depressions, each only a few centimetres across and barely a couple deep, pecked deliberately into the rock. A single further cup-mark sits on the very top of the stone. These are cup-marks, among the oldest and least understood forms of prehistoric rock art found across Ireland and Britain, and their presence here at Rossmore gives what might otherwise seem like a plain field monument an altogether more enigmatic quality.

The stone stands on elevated ground with open views in every direction, a positioning that recurs often with standing stones and may reflect concerns about visibility, either of the monument from the landscape, or of the landscape from the monument. It is oriented along a north-north-east to south-south-west axis, a detail that may or may not be intentional, though alignments of this kind are frequently noted in the study of prehistoric standing stones. The cup-marks themselves, documented by Dunne and Dennehy in 1999, are concentrated on the SSW face, which raises the possibility that they were placed in deliberate relationship to the stone's orientation, though what that relationship meant to the people who made them remains genuinely unknown. Cup-marks appear across a wide span of prehistory in Ireland, and no consensus has emerged about their function or meaning.