FitzGerald, Thomas fitz Maurice (1261–98), lord of Decies and Desmond, deputy justiciar of Ireland, was the only son of Maurice fitz John FitzGerald, the eldest son of John fitz Thomas FitzGerald (qv); his mother was Matilda, the daughter of David of Barry. Thomas became heir to the lordship of Decies and Desmond after the deaths of his father and grandfather at Callann in July 1261. His minority and wardship were granted to Thomas of Berkeley, though in practice the lordship was maintained by his uncles Gilbert fitz John and Maurice fitz John. After he came of age in 1282 he was retained by Thomas de Clare (qv), lord of Thomond, who brought him to the attention and favour of Edward I. That year he wrote to the English chancellor proposing to postpone rendering personal homage and fealty on account of local difficulties, promising to satisfy the king regarding his marriage, and asking for his inheritance to be restored. The following year fitz Maurice, his mentor Clare, Maurice fitz Maurice FitzGerald (qv), a number of Desmond barons, and Domnall Ruad MacCarthy (qv) raided and plundered the Mhic Carthaigh (MacCarthys) of Carbery.
Later in 1283 fitz Maurice accompanied Clare to England and probably toured with the royal party around Wales (1283–4). He remained in England until 1291, apparently returning to Ireland only once, in 1290, to oversee his lands. In February 1284 the king accepted his homage for his inheritance in Thomond, in May he was given a conditional grant of the lordship of Decies and Desmond (concerns had been raised over the manner in which his grandfather had entered the lordship), and in June he received custody of the king's prison at Cork and the royal castle at Dungarvan. A long-running suit brought against him by the crown in 1285 over his inheritance was finally resolved on 6 February 1292; he lost his claim to hold the shrievalties of Cork and Waterford by hereditary right and he formally surrendered his rights to Decies and Desmond to the king, who regranted it at a reduced rent of 200 marks and forgave him much of his debt.
His relationship with the MacCarthys, after achieving a modus vivendi in the 1280s, had degenerated by 1288, when the justiciar had to lead an expedition against them. At the Michaelmas parliament of 1293, which heard grievances against the justiciar, William de Vescy (qv), fitz Maurice complained bitterly that Vescy had released two of his MacCarthy prisoners and received them into the king's peace for a substantial fine without reference to himself. After the death of Vescy's successor, William de Oddingeseles, fitz Maurice was appointed deputy justiciar (custos) by the king's Irish council on 19 April 1295, a post he retained until the arrival of John Wogan (qv) on 3 December that year. His first priority was to deal with the growing threat from the Irish in the Wicklow mountains. He summoned the royal service to meet at Castledermot and succeeded in persuading Richard de Burgh (qv), earl of Ulster, and John fitz Thomas FitzGerald (qv) to attend with their respective retinues. By the early summer he had forced the MacMurroughs, the O'Tooles, and the O'Byrnes to terms, extracted hostages for good behaviour, and levied fines for the destruction they had caused. He then proceeded to make a judicial eyre throughout the south and south-west, concentrating on Cork, Kerry, Limerick, and Waterford, no doubt using the occasion to enforce his own authority; he was probably behind the initiative to move the provincial mint from Waterford to Cork. Though summoned in October 1295, he was one of the few Anglo-Irish magnates who did not take part in the Scottish expedition in 1296. And in May 1297 he was requested to campaign in Gascony.
He married Margaret, the daughter of Thomas of Berkeley and Joan, the daughter of William de Ferrers, earl of Derby, with whom he had three sons: Thomas, the eldest, who died before April 1309, Maurice fitz Thomas FitzGerald (qv), who became the first earl of Desmond, and John. Fitz Maurice died on 4 June 1298 at Knockainy in Co. Limerick and was buried with his father and grandfather in the Dominican friary at Tralee. His widow married Reginald Russel shortly afterwards without royal licence and was heavily fined. The wardship of his heir was awarded to John fitz Thomas FitzGerald.