The title of Earl of Desmond has been held historically by lords in Ireland, first as a title outside of the peerage system and later as part of the Peerage of Ireland.
The original Barony of Desmond in the province of Munster was held by descendants of Thomas FitzMaurice FitzGerald, Lord of O'Connelloe. Thomas was a younger son of Maurice FitzGerald, Lord of Lanstephan, a key supporter of Lord Pembroke ("Strongbow") in his 1169 invasion of Ireland. Maurice was the founder of the Cambro-Norman FitzGerald dynasty in Ireland. Being descended from a younger son of Maurice FitzGerald, the House of Desmond was a cadet branch of the Irish Geraldines; the senior branch, the House of Kildare, ancestors of the Dukes of Leinster, was founded by Thomas's elder brother, Gerald FitzMaurice FitzGerald, 1st Lord of Offaly.
Thomas's son, John FitzThomas FitzGerald, became the first Baron Desmond upon receiving, for his homage and service, a grant in 1259 of the lands of Decies and Desmond from Prince Edward of England. Before passing to Edward, these lands had been held by Thomas FitzAnthony, the father of John's wife Margery FitzAnthony.
The title Earl of Desmond was first created for Maurice FitzGerald, 4th Baron Desmond in approximately 1329. Over time, according to English sources, the FitzGerald family became highly assimilated to the local Irish culture. The final Earl of Desmond of this creation was Gerald FitzGerald, the 15th (or, by some counts, the 16th) Earl. The FitzGeralds had resisted the Reformation of King Henry VIII and, after the failure of the first and second Desmond Rebellions, the 15th Earl was defeated and killed by forces loyal to Queen Elizabeth I on 11 November 1583. His title, along with the enormous estates of his family, were forfeit to the English Crown. His nephew, James FitzThomas FitzGerald, attempted to regain control of both during the Nine Years War, but he was captured by the English and executed in 1603.
The second creation was in 1600 for James FitzGerald, who was also created Baron Inchiquin. These titles became extinct on his death the next year.
The third creation was in 1619 for Richard Preston, 1st Lord Dingwall, who was also created Baron Dunmore. On his death in 1628 the Earldom and Irish Barony became extinct while the Scottish Lordship of Parliament passed to his daughter Elizabeth Preston, who married the 1st Duke of Ormonde.
The fourth creation was in 1628 for George Feilding, 1st Viscount Callan. For information on this creation, see the Earl of Denbigh.