The slab measures roughly 45 centimetres high by just over a metre wide, its raised black-letter script worn but still legible. It is not original to this bridge. It was recycled, set into the stonework of a later structure after its first home had either crumbled or been demolished, and where exactly it began its life remains unknown.

What the inscription preserves is a memorial to a bridge-building act of some personal significance. As transcribed by a nineteenth-century antiquarian named Brennan, writing in 1854 to 1855, the text reads in part: '….cobi Baronis Dunboyne defuncti viri hunc pontem posuit uxor filia superstes Margareta De Brien comitis Thumoniae Ora viator.' Translated loosely, this records that Margareta De Brien, daughter of the Earl of Thomond and wife of a deceased Baron of Dunboyne, built a bridge in her husband's memory and asks the passing traveller to pray. The missing portion from the upper right corner, where the stone is broken away, would have completed the baron's name, most likely ending in something like Jacobi. The Dunboyne family held Drangan Castle, whose remains stand roughly 250 metres to the north-west, and Brennan noted that the plaque was said to have come from an older bridge that once served as the private entrance to the castle's demesne. The image of a widowed noblewoman commissioning a bridge as both a practical structure and a devotional act was not unusual in medieval Ireland, but the survival of the dedicatory plaque, however displaced, is considerably rarer.

The stone is built into the base of the southern parapet of the bridge at Knockroe, low enough that it requires a deliberate look rather than a casual glance from the roadway above. The black-letter script, a style of lettering common in formal inscriptions of the late medieval period, remains legible despite the wear, and the Latin text rewards anyone willing to crouch down and read it slowly.