In Dublin, whither he proceeded with Mountjoy, he heard of the accession of King James, at whose court he presented himself in June accompanied by Rory O'Donnell, who had become chief of the O'Donnells after the departure of his brother Hugh Roe.
Warned, however, that his arrest was imminent, and possibly persuaded by Rory O'Donnell (created earl of Tyrconnel in 1603), whose relations with Spain had endangered his own safety, Tyrone resolved to fly from the country.
His father Patrick Sarsfield married Anne, daughter of Rory (Roger) O'Moore, who organized the Irish rebellion of 1641.
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.
He married his cousin Nuala, sister of Hugh Roe and Rory O'Donnell.
When Rory fled with the earl of Tyrone to Rome in 1607, Nuala, who had deserted her husband when he joined the English against her brother, accompanied him, taking with her her daughter Grania.
Rory O'Donnell, 1st earl of Tyrconnel (1575-1608), second son of Hugh MacManus O'Donnell, and younger brother of Hugh Roe, accompanied the latter in the above-mentioned expedition to Kinsale; and when his brother sailed for Spain he transferred his authority as chief to Rory, who led the O'Donnell contingent back to the north.
In 1602 Rory gave in his allegiance to Lord Mountjoy, the lord deputy; and in the following summer he went to London with the earl of Tyrone, where he was received with favour by James I., who created him earl of Tyrconnel.
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.
Rory O'Donnell, created earl of Tyrconnel, accompanied him.