Trimmed down to a plain rectangular block and pressed into service as building material, it is easy to overlook entirely, the kind of object a visitor walks past without a second glance. Only a closer look reveals what it actually is: a fragment of a funerary slab that predates the tower by five or six hundred years, its surface still bearing a carved cross whose arms end in trefoil terminals, those small three-lobed decorative finishes that appear frequently in medieval ecclesiastical stonework. The top of the cross has been cut away, lost when the slab was reshaped to fit its new position.

The slab dates from the 13th or 14th century and is made from fossiliferous limestone, a stone formed from the compressed remains of ancient marine organisms, whose characteristic texture was widely used in medieval Kilkenny. It originally marked a grave, though no inscription survives to identify whose. By the time St Mary's parish church gained its present tower in 1820, the slab had evidently been detached from any burial context and was available as a convenient piece of dressed stone. Whoever incorporated it into the west wall of the new tower either did not recognise its age and significance, or simply did not consider that a reason to spare it from reuse. That kind of pragmatic recycling of earlier stonework was common in Irish ecclesiastical building across many periods, and it means that fragments of medieval craftsmanship sometimes survive, inadvertently preserved, within much later structures.