Fitzgerald, Thomas (1454–1534), 12th earl of Desmond , was third son of Thomas FitzGerald (qv), 8th earl, and his wife Ellice, daughter of William Barry, 8th Lord Barrymore. In April 1493 he and his older brother Maurice (qv), the 10th earl, were pardoned for their support of the Yorkist claimant to the throne of England, Perkin Warbeck (qv). In 1498 he joined with his brother-in-law Cormac Óg Láidir Mac Carthaigh (qv), later 9th lord of Muskerry, in killing Eogan Mac Teige MacCarthy, 7th lord of Muskerry, and Philip O'Sullivan Beare. He appears to have supported the coalition of forces organised by the 10th earl's son and heir James fitz Maurice FitzGerald (qv), later 11th earl, against John of Desmond in 1516. After the death of his brother Maurice in 1520, however, Thomas disputed the succession with fitz Maurice. In September of that year, with the support of his constant ally Cormac Óg Láidir, Thomas inflicted a heavy defeat on fitz Maurice, who was reported to have suffered the loss of some 2,000 men. The 11th earl survived however, and in December Thomas, again with the help of Cormac Óg Láidir, joined with Piers Butler (qv), earl of Ossory, in besieging him in Dungarvan, Co. Waterford. Fitz Maurice died 18 June 1529, and Thomas succeeded as 12th earl at the age of 75.
His primary concern thereafter was to secure the succession of his grandson and heir, another James fitz Maurice (qv), 13th earl de jure, whose legitimacy was in question because Thomas's son Maurice had married his first cousin without a papal dispensation. In October 1529 Thomas granted in fee all his possessions in Co. Waterford to John, lord of the Decies, and his son Gerald MacShane, partly in repayment for their support against the 11th earl, and partly to ensure their future support for his grandson. He also kept his alliance with Cormac Óg Láidir alive by arranging the marriage of his grandson to Cormac's daughter Mary. In May 1532 he submitted to the crown and, despite rumours to the contrary, appears to have remained aloof from the rebellion of Lord Offaly Silken Thomas (qv), in 1534. He died at Rathkeale in that year, and was buried at Youghal. He married first Sheila (Gille), sister of Cormac Óg Láidir, who was still living in 1505. Their son Maurice died of plague at Jerpoint, Co. Kilkenny, in 1529. Thomas's second wife was Katherine, daughter of John Fitzgerald, lord of the Decies, who was to become the legendary ‘old countess of Desmond’ (qv).