Whether by accident or design, the coat-of-arms set into the wall of St. Johnstown tower house does not display the correct arms of the St. John family. Instead, the quartered shield carries three fish in the first and fourth quarters, most probably the hackeds associated with the Hackett family, and two rows of large ermine spots in the second and third. It is a puzzle that has kept heraldic scholars occupied, and the plaque is more interesting for it.

The plaque is rectangular, cut from limestone and set within a chamfered recess that splays outward to follow the battered base of the tower wall, measuring roughly 82 centimetres high and 86 wide. Around the shield, carved in a mixture of Lombardic and Roman script typical of the sixteenth century, runs a Latin inscription that continues onto the chamfered surround: ROBERT DE SANCTO IOHANNE DE CVOLAGH LISMAINAN SCADANSTOWNE ET TOCIUS PLEBIS ILLIUS FECIT ME, meaning Robert St. John, Lord of Cuolagh, Lismainan, Scadanstown and all of the people of that district made me. By 1640, Robert St. John of St. Johnstown was recorded as proprietor of the site, and the Civil Survey of 1654 to 1656 described the structure as a castle with a bawn, the bawn being a defensive enclosure wall around the tower, which was then still in repair. One compelling theory about the odd heraldry is that the shield does not represent the family arms at all, but rather a coat associated with the lordship itself. The fish, most likely herrings, may be a visual reference to Scadanstown, the earlier name for the settlement, scadán being the Irish word for herring. If so, a carver working in stone encoded a piece of local place-name history directly into what looks, at first glance, like a straightforward display of family pride.