Through his first wife, he succeeded to the domains of her father, which were confirmed to him by Prince Edward, 1260.

By virtue of his royal seignory as a Count Palatine, he created 3 of his 4 sons by his second wife knights. Betham and Russel state that they were created by Edward III at the Battle of Hallidon Hill, 19 Jul 1333, but much controversy has taken place over the date and creation of these titles. The descendants of the three Knights have been styled as such in Acts of Parliament, patents under the Great Seal, and all legal proceedings up to the present time.

He was killed 1261 at Callan, Co Kerry, in a battle against the McCarthies, and was buried in the Dominican Abbey at Tralee, which he had founded that year.

From A Concise History of Ireland by Maire and Conor Cruise O'Brien, p 47:
The Hiberno-Norman lords, by contrast, throve. [in the 1300's] The Irish resurgence had made them indispensable to the Crown and the creation of the three great earldoms, Desmond (Fitzgerald), Ormond (Butler), and Kildare (Fitzgerald), towards the middle of the century sealed the King's admission of this fact, but the forfeiture and outlawry only very shortly afterwards of the first Earl of Desmond demonstrates how dubious their surety could be. Indeed, the King's subjects in the colony complained only less of the exactions of the great earls than of the depredations of their Irish counterparts. . . . From the very outset the Norman knights had intermarried with the Irish and were quick to grasp the political usefulness of a custom such as fosterage, under which the children of noble houses were brought up from infancy in other noble families with whom it was desirable to cement alliances.

From Irish Roots, 1995, #3, p12, 'Surnames of County Kerry': The ancestor of the Earls of Desmond, John Fitz Thomas, also acquired from Henry II large grants in Kerry, which were augmented by Prince John in 1199. Henceforward, the family of Fitzgerald exercised a predominant authority in Kerry.

From "Complete Peerage England, Scotland, Ireland, Great Britian" by George E Cokayne, CS421.G2 1982, v IV, page 233, Desmond:
John FitzThomas, son and heir of Thomas FitzMaurice (who died in 1213 or 1214), by Sabina or Sadhbh, his wife. The younger son of Morice fitz Gerald, one of the conquerors of Ireland, his elder brothers being William of Naas, and Gerald of Offaly. On 6 Sep 1199 the King gave him 5 knights' fees in the tuath of Eleuri in the cantred of Fontemel, and 5 knights' fees in the tuath of Huamerith in Thomond, on the Shannon, to hold by the service of 3-1/3 knights, in fee, and a burgage within the walls of Limerick, to hold by the service of 12d. a year. (Charter Roll, I Joh., p. I, m. 13). His widow made a fine with the King in Poitou [i.e., between Feb and Oct 1214] for the custody and marriage of his son and heir (Close Roll, 16 Joh., p. 2, m. 9), and she afterwards paid £40 for liberty to remarry (Fine Roll, 17-18 Joh., m.9). Having married Margery, daughter and co-heiress of Thomas FitzAnthony, he obtained, 23 May 1251, a provisional grant of £25 a year, in compensation for his purparty of Decies. Patent Roll, 35 Hen. III, m. 8. On 7 Nov 1259 Edward, the King's eldest son, at that time under age, gave him, for his homage and service, all the lands of Decies and Desmond, then in the hand of the said Edward, and which had been held by Thomas fitz Anthony, father of Margery wife of the said John, and the custody of the castle of Dungarvan, in fee, of which the said Thomas had been invested by the gift of King John, and of which he had died seized, to hold, to John and his heirs, at a rent of 500 marks a year, by the service of 1/10 of a knight's fee. With the proviso that if there was common war in Ireland, or if the King had certain suspicion of John or of his heirs, quod absit, the castle of Dungarvan should be rendered up to the King, to be held by him as long as the war should last, or till the suspicion was allayed. And if the premises descended to an heir female, then the King might hold the said castle till an heir male succeeded or the same heir female should marry. Gascon? Roll, 35 Hen. III, m.5: Patent Roll [I.], antiquissime, d, no. 32: Judiciary Rolls [I.], vol. i, p. 153. The Justiciar refused to give him seizin, saying that the Lord Edward had been deceived, and assigned him to appear before Edward's Council: whereupon he took seizin himself of these lands in the first week of Lent 1259/60 and held them till his death. He died 23 Jul 1261, being slain in a conflict with the MacCarthys of Carbery, at Callan in Kerry. Annals of Multifernam, p. 14: Annals of Ulster, vol ii, p. 330: Annals of Loch Ce, vol. i, p. 438; Clyn, Annales, p. 8: Annals of Ireland, p. 316. These give the year only, 1261. Callainn Gleanna O'Ruachtain, the site of the battle, is about 5 miles east of Kenmare. He was buried in the Dominican Friary at Tralee in that county which he had founded. Red Book of the Earls of Kildare, Hist. MSS. Com., 9th Report, part ii, p. 288. John FitzThomas is said to have had by Honora (some call her his second wife), daughter of Fedhlim O'Conor Donn of Connaught, 4 sons, ancestors of the White Knights, the Black Knights or Knights of Glin, the Green Knights or Knights of Kerry, etc.

He was Lord of Decies and Desmond and a Count Palatine in the year 1259. By virtue of the latter royal position, he created three of his sons by his second wife Honora, hereditary knights; and thus originated the titles of the 'White Knight.' the 'Knight of Glyn' and the 'Knight of Kerry. He was also father of a son by his first wife, who was called, Osborn Fitz Gerald. This son was also denominated by the Welsh herald, Osborn Wyddel (Osborn, the Irishman).