It is a keystone, the wedge-shaped central block that once locked an arched gateway into place, and it carries on its face a small but densely composed scene: two family coats of arms, an elaborate carved frame enclosing a pair of initials, and above that a human head cut in relief, gazing out from the stone with the particular self-possession of seventeenth-century commemorative carving.

The plaque originally spanned the arch of a gateway in Marsh's Street, and it records the union of two Kilkenny families. The arms of the Walsh and Brenan families appear on either side, with the initials "J.W." and "C.B." at the centre, standing for John Wailsh and Catrin Brenan, whose names are spelled out in full below the date 1645. The spelling of both names reflects the orthographic habits of the period, when Irish and English conventions sat loosely alongside one another. At some later point, the date 1794 was added to the stone, incised by a different hand for reasons that are no longer recorded. Whether that second date marks a repair to the gateway, a change of ownership, or simply someone's impulse to leave a mark, it now reads as a kind of postscript carved onto an already complete document, the two dates separated by a century and a half of unrecorded life.