Hugh Dubh had been king (Rí) of the O'Donnells during one of the bitterest and most protracted of the feuds between his clan and the O'Neills, which in 1491 led to a war lasting more than ten years. He left his son to rule Tyrconnell (spelled in Irish as Tír Chonaill), though still a boy, when he went on a pilgrimage to Rome about 1511. On his return from Rome in broken health after two years' absence, his son Manus, who had proved himself a capable leader in defending his country against the O'Neills, retained the chief authority. When Hugh Dubh appealed for aid against his son to the Maguires, Manus made an alliance with the O'Neills, by whose assistance he established his hold over Tyrconnell. But in 1522 the two great northern clans were again at war.
Conn Bacach O'Neill, 1st Earl of Tyrone, was determined to bring the O'Donnells under his rule. Supported by Munster and Connacht, and assisted also by English contingents and by the MacDonnells of Antrim, O'Neill took the castle of Ballyshannon, and after devastating a large part of Tyrconnell he encamped at Knockavoe, near Strabane. Here he was surprised at night by Hugh Dubh and Manus O'Donnell, and routed with the loss of 900 men and an immense quantity of booty in the Battle of Knockavoe. Although this was one of the bloodiest ever battles between the O'Neills and the O'Donnells, it did not end the war; and in 1531 O'Donnell applied to the Lordship of Ireland for protection, giving assurances of allegiance to King Henry VIII.
In 1537 Lord Thomas Fitzgerald and his five uncles were executed for rebellion in Leinster, and the English government made every effort to capture Gerald FitzGerald, the heir to the earldom of Kildare, a boy of twelve who was in the secret custody of his aunt Lady Eleanor McCarthy. This lady, in order to secure a powerful protector for the boy, accepted an offer of marriage from Manus O'Donnell, who on the death of Hugh Dubh in July 1537 was inaugurated as "The O'Donnell". Conn O'Neill was a relative of Gerald Fitzgerald, and this event accordingly led to the formation of the short-lived Geraldine League, a federation including the O'Neills, the O'Donnells, the O'Briens of Thomond, and other powerful clans; the primary object of which was to restore Gerald to the earldom of Kildare, but which afterwards aimed at the complete overthrow of English rule in Ireland.
Chief of the O'Donnells
In August 1539 O'Donnell and Conn O'Neill were heavily defeated by the lord deputy at Lake Bellahoe, in County Monaghan, which crippled their power for many years. In the west Manus made unceasing efforts to assert the supremacy of the O'Donnells in north Connacht, where he compelled O'Conor Sligo to acknowledge his over-lordship in 1539. In 1542 he went to England and presented himself, together with Conn O'Neill and other Irish chiefs, became a Protestant, and recognising Henry VIII, who promised to make him Earl of Tyrconnell, though he refused O'Donnell's request to be made Earl of Sligo. The assimilation process was known as "surrender and regrant".
In his later years Manus was troubled by quarrels between his sons Calvagh and Hugh MacManus; in 1555 he was imprisoned by Calvagh, who deposed him from all authority in Tyrconnell, and he died in 1564. Manus O'Donnell, though a fierce warrior, was hospitable and generous to the poor and the Church. He is described by the Four Masters as "a learned man, skilled in many arts, gifted with a profound intellect, and the knowledge of every science." At his castle of Portnatrynod near Strabane he supervised, if he did not actually dictate, the writing of the Life of Saint Columbkille in Irish, which is preserved in the Bodleian Library (Rawlinson B 514) at Oxford. He was also a poet and many of his poems, written in Irish, survive.
Manus was married several times. His first wife, Joan O'Reilly, was the mother of Calvagh, and two daughters, both of whom married O'Neills; the younger, Margaret, was wife of the famous rebel Shane O'Neill. His second wife, Hugh's mother, by whom he was ancestor of the Earls of Tyrconnell (see below), was Judith, sister of Conn Bacach O'Neill, 1st earl of Tyrone, and aunt of Shane O'Neill.
- Rev. Richard Henebry: The Life of Columcille (ascribed to Magnus O'Donnell). Text from Rawlinson B. 514, with translation. (Zeitschrift für celtische Philologie. III., IV., V., 1901-05).
- Four Masters, Annals of Ireland by the: Translated and Edited by John O'Donovan. Dublin, 1856.
- This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "article name needed". Encyclopædia Britannica (11th ed.). Cambridge University Press.