It measures just 33 centimetres long and 44 centimetres wide, modest enough to pass unnoticed entirely. What makes it quietly significant is its journey: the plaque was not placed here by design in any grand commemorative sense, but was salvaged from a coach house that once stood on the same ground, itself a remnant of a far older occupation of the site.

The land on which St. Mary's now stands was the location of Merrion Castle, the ancestral seat of the Fitzwilliam family from at least the fifteenth century. The Fitzwilliams were among the most prominent Anglo-Norman dynasties in the Dublin area, and their name endures across this part of the city in ways that most people encounter daily without much thought, from Fitzwilliam Square to the stretch of coastline nearby. The castle itself is long gone, absorbed and overwritten by later development, but this armorial plaque survived, presumably built into the coach house at some earlier point and then carefully relocated when Fitzwilliam Hall was constructed on that same footprint. An armorial plaque of this kind, carved with a family's heraldic device, would originally have served as a public declaration of ownership and lineage, the stone equivalent of a signature on the landscape.

The plaque is within the grounds of an active care facility, so access is not a matter of simply wandering in. Visitors with a specific interest in the site would be wise to contact St. Mary's Home in advance. The plaque itself is set into the southern gable wall of Fitzwilliam Hall, so knowing to look for that particular building within the complex is useful. Given its small dimensions, it rewards close attention rather than a glance from a distance, and the carved heraldic detail is the thing to focus on once you are standing in front of it.