Each is divided into three compartments, and in each compartment an angel, carved in false relief, holds up an armorial shield. The tablets are made from fossiliferous limestone, meaning the stone itself contains the faint impressions of ancient marine creatures, and together they measure just 35 centimetres high and 60 centimetres wide. For objects of such modest dimensions, they carry a surprisingly complicated heraldic argument.

The better-preserved of the two plaques, positioned at the western end of the wall, displays three shields whose identities have been the subject of careful but inconclusive scrutiny. The first has been tentatively linked to the FitzSimon family, though the precise connection with Trim remains unclear. The second is described by researcher Hickey as very unusual: a lion rampant crossed by a horizontal band, within a wavy border, possibly representing the de Lacy family, though this is far from certain. The de Lacys were among the most powerful Anglo-Norman lords in medieval Meath, which would make their presence here historically plausible if unproven. The third shield is the most intriguing: four lions rampant arranged in quadrants, alternating black and red on a gold field. These are the arms of Queen Philippa, wife of Edward III and mother of Lionel, Duke of Clarence. Hickey suggested the shield may actually represent the Bathe family, prominent in Dublin and Meath during the fifteenth century, who appear to have adopted or adapted the royal arms. The plaques are thought to date from that same century, and together with an associated piscina, a small stone basin used for washing sacred vessels during Mass, they likely formed a single decorative ensemble. Whether they were always at St Patrick's is itself uncertain. Hickey raised the possibility that they originated in the nearby Priory of St Mary at Trim, pointing to the possible de Lacy reference, the presence of the arms of the Virgin Mary elsewhere in the group, and records of grants made to St Mary's Abbey by both the Duke of York and Edward IV. Against this, a seventeenth-century manuscript now held in Trinity College Dublin contains drawings of the shields from the St Patrick's piscina under the heading "in ecclesia de Trim", which suggests the pieces were associated with that church at least by the 1600s, and possibly from the beginning.