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. Niall Garve made terms with the English government, to whom he rendered valuable service both against the O'Neills and against his cousin, enabling an English force to land at Derry under Sir Henry Docwra: Docwra for a time regarded Niall as an invaluable ally. 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.