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He was the eldest son of Thomas FitzMaurice, Lord OConnello (died 1271) and Rohesia de St. Michel. He is noticed in 1291 in serious dispute with William Vescy, Lord of Kildare, Lord Justice of Ireland, about whom there were many complaints of oppression and neglect of the country's defences. As champion of the complainants John FitzThomas, by then 4th Lord of Offaly (having succeeded to the title in 1287, upon the death of his uncle Maurice FitzGerald, 3rd Lord of Offaly), their paths crossed and instead of addressing the issues Vescy charged FitzThomas with minor charges of slander and libel. FitzThomas appealed to King Edward I of England, who, to examine and judge the matter impartially, summoned them both to London to hear the cases, where it appears FitzThomas had the advantage, challenging the Lord Justice to clear his name by combat, which was accepted. However, Vescy fled to France, whereupon the King pronounced Lord Offaly innocent, and settled upon him Vescy's lordships and manors of Kildare, Rathangan, &c., which had been forfeited to the Crown.
FitzThomas, perhaps inevitably, clashed with the powerful and expansionist Richard de Burgh, 2nd Earl of Ulster. Their quarrel was at its height in 1294-5 when FitzThomas captured de Burgh and imprisoned him at Lea Castle for several months "to the disturbance of the whole land". The Parliament of Ireland eventually secured de Burgh's release. FitzThomas, though he was indicted at Westminster for a number of very serious offences, obtained a royal pardon for all of them except those against de Burgh, to whom he was forced to surrender his lands in Connacht. Thereafter he and de Burgh were on reasonably amicable terms, and FitzThomas's eldest son married de Burgh's daughter Joan. 
In 1307, with his son-in-law Sir Edmund Butler, he dispersed rebels in Offaly who had razed the castle of Geashill and burnt the town of Leix. In 1312 he was sent as General at the head of an army into Munster to suppress armed Irish rebels. On 25 May 1315, Edward Bruce, brother to King Robert the Bruce of Scotland, entered the north of Ireland with 6000 men, was crowned King of Ireland at Dundalk, and wasted the country. Lord Offaly, among others, commenced vigorous sporadic warfare to resist Bruce, leaving "great slaughter" of Scots and the Irish irregulars in his service. Edward Bruce was eventually defeated and killed in the battle of Dundalk.
He had married Blanche de La Roche, daughter of John de La Roche, Lord Fermoy and Maud Waley (daughter of Henry Waley), by whom he had two sons and two daughters:
- Gerald (d.1303)
- Thomas FitzGerald, 2nd Earl of Kildare, his successor.
- Joan FitzGerald, married in 1302 to Edmund Butler, Earl of Carrick.
- Elizabeth FitzGerald, married to Nicholas Netterville, ancestor of Viscount Netterville.
- Webb, Alfred. A Compendium of Irish Biography, Dublin. M.H. Gill & Son, 1878
- Otway-Ruthven, A.J. A History of Medieval Ireland Barnes and Noble reprint New York 1993 p,211
- Lives of Illustrious and Distinguished Irishmen, (James Wills, ed.), MacGregor, Polson & Co., Dublin, 1839
- "History", Adare Village
- Cokayne, George Edward, ed. (1892). Complete peerage of England, Scotland, Ireland, Great Britain and the United Kingdom, extant, extinct or dormant (G to K). 4 (1st ed.). London: George Bell & Sons. p. 368. Retrieved 27 December 2011.
- Lodge, John, & Archdall, Mervyn, A.M., The Peerage of Ireland, Dublin, 1789, vol.1, p. 77-9, where a full transcription of Kildare's Letters Patent appears on p. 78n.
|Peerage of Ireland|
|New creation|| Earl of Kildare