FitzGerald, John (a.1377–1399) and Thomas fitz John FitzGerald (c.1386–1420), 4th and 6th earls of Desmond, succeeded in turn to the earldom but neither managed to make a lasting impact on the fortunes of their family. John fitz Gerald was the eldest son of Gerald fitz Maurice FitzGerald (qv), 3rd earl of Desmond, and his wife Eleanor, daughter of James Butler (qv), 2nd earl of Ormond. He was granted a general pardon in 1393 and was appointed sheriff of Waterford in February 1393. John was commissioned with his uncle to treat with the Gaelic lords of Munster to secure their submission to Richard II (qv), and acted as an interpreter for the king at the submissions of Diarmaid Ó Conchobhair Failghe and Art Ó Díomusaigh in April 1395. He was knighted before April 1395 and succeeded to the earldom of Desmond on his father's death (1398). However, he died while crossing the Suir in October 1399, returning from a retaliatory raid against the earl of Ormond, and was buried in the Franciscan friary at Youghal; his death left his 14-year-old son Thomas as the new earl.
In May 1400 Maurice fitz Gerald, Thomas's uncle, was granted custody of the lands of the earldom during the minority of the new earl. In 1402 James Butler (qv), 3rd earl of Ormond, led an invasion against Desmond, but it is unclear whether he was fighting Earl Thomas or his uncle. In March 1406 Thomas was pardoned for violations of the peace and granted all the lands held by his grandfather, the 3rd earl of Desmond, despite his still being a minor. In 1407 he joined with the deputy, Stephen Scrope (qv), and James Butler (qv), 4th earl of Ormond, to repulse an attack by the O'Carrolls and the Burkes against Callan, and in September of the same year he was made one of the justices for Co. Cork, Co. Limerick, and Co. Kerry. Thomas was banished by his uncle, James fitz Gerald FitzGerald (qv), in 1411, and went to England, where his claims to the earldom were upheld. By August 1413 he had secured royal approval to bring a force of soldiers to Munster to regain his lands, but he was captured and imprisoned by his uncle till released into the custody of the lieutenant, John Talbot (qv), Lord Furnival. According to later tradition, Thomas formally renounced his claims to the earldom in 1418, but no surviving evidence supports this belief. He then travelled to France, where he joined the king at the siege of Rouen. He served in France, probably hoping to gain support for a second attempt at retaking his lands, but died in the summer of 1420 and was buried in Paris; Henry V was said to have attended his funeral. Several reasons have been given for his expulsion from Munster: that he married an Irish peasant against the statute of Kilkenny, or that he himself was illegitimate, and that his father's legitimate heir had always been James fitz Gerald. However, these seem to be justifications for the naked usurpation of his earldom by an ambitious uncle. He left one son, Maurice, who was either illegitimate or barred from the earldom by the acceptance of James fitz Gerald as rightful earl of Desmond.