What makes it worth seeking out is what covers its upper surface: five incised crosses of varying form and depth, carved into stone that measures roughly 2.8 metres by 1.6 metres. Among them is a deeply grooved equal-armed cross, three smaller incised crosses, and a cross-in-circle motif whose shaft extends beyond the enclosing ring and ends in a flat base, a detail that gives it an almost architectural quality. Locally, the boulder is known as a mass-rock.

Mass-rocks were places where Catholic priests celebrated Mass in secret during the Penal Law era, roughly the late seventeenth and eighteenth centuries, when the public practice of Catholicism was suppressed under a series of restrictive statutes. A large flat boulder in an open, relatively hidden landscape would have served as an improvised altar, and the presence of incised crosses on such stones, whether carved in that period or earlier, is not unusual across the south-west of Ireland. Whether the crosses on this boulder predate its use as a mass-rock, perhaps belonging to an earlier medieval tradition of cross-marking sacred or boundary stones, or were added during the Penal period, is not recorded. The Iveragh Peninsula, on which Ballydarrig sits, is dense with early Christian and medieval field monuments, and a cross-in-circle motif of this kind has parallels in early medieval Irish stone carving, making the question of date genuinely open.

The boulder is not signposted, and its absence from OS maps means that finding it depends on local knowledge or a careful reading of the terrain on the lower slopes of Knocknadobar. The partially overlying field boundary is probably the easiest landmark once you are in the right area; the stone sits beneath it rather than standing proud of the landscape, which goes some way to explaining why it has remained so quietly unnoticed.