A fragment of a medieval graveslab, measuring roughly 44 centimetres by 28 centimetres, has been built into the north end of the east wall of the abbey's modern sacristy, where it functions as a quoin stone, that is, a corner stone used to reinforce the angle of a wall. It is partly obscured by pebble-dash rendering, the kind of rough external plaster coating common to Irish buildings, which means only a portion of its surface is visible at all.

What that visible surface preserves is worth pausing over. Cut into the stone is a Latin inscription reading "O DNI 1645", an abbreviated form of "Anno Domini 1645", the year of the Lord 1645. The date places the original graveslab in a period of considerable turbulence in Ireland, just before the Cromwellian campaigns that would devastate much of the country and suppress Catholic religious houses including the Augustinians. Whether the slab was already a fragment when it was incorporated into the wall, or was broken in the course of later building work, is not recorded. What is clear is that someone, at some point, saw a piece of funerary stonework as serviceable building material and used it accordingly. The practice was not unusual; dressed stone from older structures was routinely recycled, particularly in periods of disruption or poverty. But the result here is a date commemorating a specific death, or perhaps a burial, sealed into the corner of a later wall and half-plastered over.