At Castlebank in County Clare, however, a single reused block carries considerably more weight than its position suggests. It is the keystone from a mantle piece, bearing the O'Brien coat-of-arms and the date 1578, and it has been built into the gable wall beside the main entrance gate to the yard, far from whatever hearth it once crowned.

Keystones are the wedge-shaped central blocks of an arch, and in a fireplace surround they would have sat at the top of the opening, holding the whole composition in place. That this one was carved with heraldic arms points to a building of some standing. The O'Briens were among the most powerful Gaelic dynasties in Munster, and 1578 falls within a period when their leading branches were navigating the pressures of Elizabethan expansion while still commissioning substantial stonework. According to local knowledge recorded in 2014, the keystone almost certainly originated in the castle associated with the Castlebank site. At some point it was removed, perhaps during demolition or renovation, and incorporated into the fabric of the outbuilding where it remains today, heraldry and date still legible after more than four centuries.