Built into the structure as a lintel, the stone bears an incised Latin cross, its shaft running along the longer axis of the slab. The decoration is partly obscured by the surrounding masonry, which means that what was once a marker for the dead now quietly holds up a wall, its sacred geometry only partially legible to anyone who looks closely enough.

The stone in question sits within Archibold's Castle, one of several tower houses that gave Dalkey its unusual medieval character as a prosperous port town. A lintel is simply a horizontal stone or beam spanning an opening, and builders of the period were entirely practical about where such stones came from. This one, researchers believe, was recycled from a grave slab, most likely originating at the nearby church of St. Begnet's, the early medieval parish church around which much of Dalkey's older history clusters. The tapering of the slab towards the base of the cross is what gives the game away; it is the characteristic form of a grave marker rather than a decorative architectural element. The detail is noted by Mason in 1983 and repeated in a 2009 edited volume by K. Swords, suggesting the identification has been consistent across several decades of scholarship. The compilation of the formal record was carried out by Geraldine Stout and Padraig Clancy.

Archibold's Castle stands on the south side of Castle Street and is one of the more accessible of Dalkey's surviving tower houses, though the cross-inscribed stone itself requires some patience to properly observe, given that the decoration is partially hidden within the construction. Visitors who know to look for it will find the incised cross only partly revealed, the rest swallowed by centuries of building. St. Begnet's Church and its graveyard are close by and worth pairing with a visit, since they provide the likely origin point for the slab and give a clearer sense of the early Christian context from which it was taken and repurposed.