Calvagh O'Donnell (; 26 October 1566), eldest son of Manus O'Donnell, was an Irish King of Tyrconnell of the mid-16th century. He was king and chief of the O'Donnell dynasty based in Tyrconnell in western Ulster. He is best known for his conflict with Shane O'Neill – a dispute that involved the intervention of the English government in Ireland on Calvagh's side.
Life
Calvagh O'Donnell was the eldest son of Manus O'Donnell by his first wife, Joan O'Reilly. In the course of a quarrel with his father and his half-brother Hugh, Calvagh sought aid in Scotland from the Campbells, who with access to Scottish royal artillery were able to assist him in deposing Manus and securing the now very divided lordship of Tyrconnell for himself.
Hugh then appealed to Shane O'Neill, chief of the neighbouring O'Neill dynasty, to restore him at Calvagh's expense. Shane accordingly invaded Tyrconnell at the head of a large army in 1557, desiring to make himself supreme throughout Ulster, and encamped on the shore of Lough Swilly. and perhaps as early as 1540, Calvagh was married to a daughter of the 4th Earl of Argyll whose name may have been Janet and who may have been the mother of his son, Conn. By 30 May 1561 Calvagh was married to his last wife, Catherine Maclean, widow of the 4th Earl.
thumb|Killydonnell Friary at the western shore of [[Lough Swilly where Calvagh and his wife were captured in 1561 by Shane O'Neill]]
Calvagh was then recognized by the English government as lord of Tyrconnell; but in 1561 he and his wife were kidnapped by Shane O'Neill in the Franciscan friary of Killydonnell. His wife had previously been the wife of the Earl of Argyll, was kept by Shane O'Neill as his mistress and bore him several children.