Background
He was the son of Conn O'Donnell, the son of Calvagh O'Donnell the ruler of the lordship of Tyrconnell.
He was the son of Conn O'Donnell, the son of Calvagh O'Donnell the ruler of the lordship of Tyrconnell.
He is best known for siding with the English against his kinsman Hugh Roe O'Donnell during the Nine Years' War in the 1590s. While Red Hugh O'Donnell was engaged in the Nine Years' War against the English, Niall Garve exploited the political situation to his own advantage. But in 1601 he quarrelled with the Lord Deputy of Ireland, who, though willing to establish Niall Garve in the lordship of Tyrconnell, would not permit him to enforce his supremacy over Sir Cahir O'Doherty in Inishowen.
Nonetheless the same year he led an Anglo-Irish force that captured Donegal. It was then laid siege to by the rebels, but Niall Garve oversaw a successful defence. After the departure of Hugh Roe from Ireland in 1602, Niall Garve tried to seize the chieftainship, and was "inaugurated" as the 25th O'Donnell in 1603, but without the full required support of the derbfine (electoral kinship group).
To find a solution, Niall Garve, and Hugh Roe's brother Rory went to London in 1603, where the privy council endeavoured to arrange the family quarrel. As a result, King James I of England granted some lands to Niall Garve, but raised Rory to the peerage as Rory O'Donnell, 1st Earl of Tyrconnell, and also granted him the territorial Lordship of Tyrconnell. Niall Garve was later accused of turning against the Crown.
When Sir Cahir O'Doherty launched an uprising by burning Derry in 1608, Niall Garve was charged with complicity in the ensuing O'Doherty's Rebellion. She was the subject of an Irish poem, of which an English version was written by James Mangan from a prose translation by Eugene O'Curry. A bhean fuair faill ar an bhfeart
Thruaigh liom a bhfaghthaoi d'éisdeacht
Dá mbeath fian Gaoidheal ad ghar
Do bhiadh gud chaoineadh cognamh…
'O woman that hast found the tomb unguarded,
pitiful to me the number thou findest to listen;
were the soldiery of the Gaels at thy side there would be help with thy keening.