But the small Latin cross cut into its south-western face, and the fragmentary inscription on a now-broken upright slab placed on top of it, tell a more pressured story. The inscription reads, in part: "ALTAR OF PENAL DAYS".

Mass-rocks are the physical remnants of Catholic worship conducted in secret during the Penal Law era, roughly the late seventeenth and early eighteenth centuries, when legislation barred Catholics from practising their faith openly, building churches, or receiving the sacraments from a registered priest. In response, congregations gathered outdoors, often in remote or sheltered spots, where a flat rock or a roughly constructed stone surface served as an improvised altar. The Kilnadur example, in County Cork, follows this pattern closely. The altar itself is modest in scale, measuring under a metre in length and just over half a metre wide, resting on a stone-built base rather than the bare ground. The cross inscribed on its outer face is similarly unassuming, roughly sixty centimetres by forty, cut directly into the sandstone. What makes this particular site notable is the presence of the commemorative upright slab, now broken, which suggests that at some later point the community deliberately marked the spot as a place of historical memory, not merely a functional object left behind in a field.