The stone is roughly three square metres across, sub-triangular in shape, and what has been carved into its smoothed but weathered upper face is easy to miss: faint cup-and-ring motifs, an oval depression, and picked lines that catch the light only at certain angles.

Cup-and-ring marks are among the most widespread and least understood forms of prehistoric rock art found across Ireland and Britain. The basic form is exactly what the name suggests, a small circular hollow, the cup, surrounded by one or more concentric rings carved into the rock surface. At Coomshanna, two variants on this motif appear together, suggesting the stone was not decorated in a single moment of impulse but may have accumulated marks over time, or was worked by hands familiar with more than one local tradition. What the motifs meant to the people who carved them remains genuinely unknown. The Coomshanna boulder does not stand alone in the landscape, however. Clustered nearby are the collapsed remains of more than ten hut sites, built close together in various forms, alongside low drystone walls. Drystone construction uses no mortar, the stones relying on their own weight and fit to hold together. The presence of this settlement evidence beside the decorated boulder raises the obvious question of whether the two are related, though the chronological relationship between prehistoric rock art and later habitation in upland Kerry is rarely straightforward.

The boulder sits in open upland pasture, and the motifs are described as faintly visible, which means that flat, raking light, the kind found on overcast days or in the lower sun of morning and evening, will make considerably more of the carvings than direct overhead light ever will. The collapsed hut sites nearby are worth looking at slowly; the variety of their forms, some round, some less so, some barely legible as structures, reflects how these marginal upland areas were used and reused across long stretches of time.