Arms: Ermine, a saltire Gules.
Plain English: An ermine shield bearing a red diagonal cross.
The family
The FitzGeralds, or Geraldines, were one of the great Anglo-Norman families of medieval Ireland. Descended from the family of Gerald of Windsor, they divided into several powerful branches, of which the Earls of Kildare and Earls of Desmond became the most prominent.
During the fifteenth and early sixteenth centuries the Desmond FitzGeralds exercised extensive power in southern Ireland, while the Kildare branch became increasingly influential in the government of Ireland.
The arms
The arms Ermine, a saltire Gules — an ermine field crossed by a red saltire — belonged particularly to the FitzGerald Earls of Desmond.
The related Kildare branch bore the similar Argent, a saltire Gules: a red saltire on a plain silver or white field. The ermine therefore provides an important distinction between the two branches.
Salisbury and Wiltshire
Unlike Stourton, Hungerford, Audley and Beauchamp, no close connection between the FitzGeralds of Desmond and medieval Salisbury or Wiltshire has yet been established.
This makes the proposed identification on the St Thomas’s ceiling interesting, but also gives particular reason for caution.
On the St Thomas’s ceiling
Arms identified as FitzGerald appear at:
Both RCHME and Buckeridge identify these shields as FitzGerald, although RCHME marks the identification as uncertain. The surviving heraldry is consistent with Ermine, a saltire Gules, but the identification should therefore be regarded as probable rather than certain.
Sources
RCHME and Buckeridge — identifications of the St Thomas’s ceiling heraldry.
Heraldry Society, Fenwick Roll — FitzGerald arms of Desmond and Kildare.