John FitzThomas, 1st Baron Desmond (died 1261) was the son of Thomas Fitzmaurice, Lord OConnello by his wife Ellinor, daughter of Jordan de Marisco, and sister of Geoffrey de Marisco, who was appointed justiciar of Ireland in 1215.

[citation needed] In July 1261 the three Gaelic clans joined to face the Normans at the Battle of Callann and won a complete victory.

John FitzGerald was succeeded by his grandson, Maurice's son Thomas FitzMaurice, 2nd Baron Desmond.

[7] John was the father of Maurice FitzJohn, who died with him at the Battle of Callann, and of Osborn Fitzgerald, founder of the House of Corsygedol.

[3][8][9] John was succeeded in his baronial title by Maurice's son, Thomas FitzGerald, 2nd Baron Desmond.