Charged with complicity in Cahir O'Dogherty's rebellion in 1608, Niall Garve was sent to the Tower of London, where he remained till his death in 1626.
But the arrangement between Rory and Niall Garve insisted upon by the government was displeasing to both O'Donnells, and Rory, like Hugh Roe before him, entered into negotiations with Spain.
Niall Garve made terms with the English government, to whom he rendered valuable service both against the O'Neills and against his cousin.
After the departure of Hugh Roe from Ireland in 1602, Niall Garve and Hugh Roe's brother Rory went to London, where the privy council endeavoured to arrange the family quarrel, but failed to satisfy Niall.
But in 1601 he quarrelled with the lord deputy, who, though willing to establish Niall Garve in the lordship of Tyrconnel, would not permit him to enforce his supremacy over Cahir O'Dogherty in Inishowen.
Niall Garve O'Donnell (1569-1626), who was incensed at the elevation of his cousin Hugh Roe to the chieftainship in 1592, was further alienated when the latter deprived him of his castle of Lifford, and a bitter feud between the two O'Donnells was the result.
The government now sent Sir Henry Docwra to Derry, and O'Donnell entrusted to his cousin Niall Garve the task of opposing him.