It is the keystone of a joggled arched mantelpiece, meaning it once formed the central locking piece of a fireplace surround built with interlocking, angled joints rather than simple horizontal courses. Carved into it is a Latin inscription: "AD 1596 DONALDUS CARTI ET MARGARETA GERALD FECERUNT", which translates roughly as "Donal McCarthy and Margaret Fitzgerald made this tower, AD 1596". The stone is a rare survival of a named commission, the kind of personal record that rarely outlasts the building it came from.

The stone did not originate at Upton House. According to W. Hawkes, writing in 1951, it was moved there from Kilbrittain Castle, a McCarthy stronghold some distance to the south-west in west Cork. The inscription places its making in 1596, a turbulent decade in Munster during which the McCarthy lordships were under severe pressure from the Elizabethan administration. That Donal McCarthy and Margaret Fitzgerald, whose family name in Latin form "Gerald" points to a Geraldine connection, chose to mark a building project with their names and the year suggests a deliberate act of ownership and permanence, even as the political ground shifted beneath them. Coakley, who published a translation of the inscription in 1920, identified the two figures but the stone itself remains the most direct evidence of their collaboration, chiselled into a piece of Cork limestone and carried intact through several centuries and at least one change of address.