Irish Roman Catholic nobleman who led one of the three major Irish rebellions against English rule under Queen Elizabeth I.

The son of James FitzJohn, 13th earl of Desmond, he succeeded to his father's title and lands in Munster (southwestern Ireland) in 1558 and was soon embroiled in territorial disputes with Thomas, 10th earl of Ormonde (Ormonde, Thomas Butler, 10th earl of). The two rivals pleaded their cases before Elizabeth in 1560, but Desmond's manner so provoked the Queen that she had him imprisoned for a short time.

Returning to Ireland in 1564, Desmond quickly took up arms against Ormonde; early in 1565 he was wounded and captured in battle at Affane. The Queen then decided the feud in favour of Ormonde, and when Desmond failed to abide by the agreement he was arrested in 1567 and kept in honourable detention in Dublin and London for six years. During this interval Desmond's cousin James (Fitzmaurice) Fitzgerald (Fitzgerald, James Fitzmaurice) launched a rebellion against the English but came to terms in February 1573, shortly before the Earl's release.

Then in July 1579 Fitzmaurice invaded Ireland with a small force of Italians and Spaniards, backed by the Pope and King Philip II of Spain. Desmond did not join them until after Fitzmaurice was killed in August. Desmond took charge of the papal army and appealed to the Irish lords to join in the defense of Catholicism against the English Protestants. The English brutally suppressed the insurgents and then left Ormonde to hunt down Desmond. The death of the Earl in a minor skirmish brought to a close a conflict that had devastated Munster.

Whether Gerald Fitzgerald is designated the 14th or 15th earl depends on an earlier disputed succession.