Manus's father, Hugh Dubh (pronounced in Ulster Irish as 'Hugh Doo'), had been Rí (king) 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.

On his return from Rome (via England, where he was knighted by King Henry VIII[4]) in broken health after two years' absence, Manus, who had proved himself a capable leader in defending his country against the O'Neills, retained the chief authority.

"[1] When Sir Hugh Dubh O'Donnell, as he was now, 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.

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 Sir 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.

Conn O'Neill (later Earl of Tyrone) was a relative of the young Lord Kildare, 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 young Lord Kildare to his lands, titles and properties, but which afterwards aimed at the complete overthrow of English rule in Ireland.

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.