After initially supporting his rival and cousin the lord of Tyrconnell Hugh Roe O'Donnell and his ally Hugh O'Neill, Earl of Tyrone during Tyrone's Rebellion, Niall switched sides in 1600 to support the Crown forces, in particular the expedition landed under Sir Henry Docwra at Derry.
Conn and two other brothers, Hugh Boy and Donal also defected bringing with them as many as a thousand warriors by some estimates.
[3] They assisted the Crown's capture of Lifford, a key O'Donnell stronghold, to the outrage of Hugh Roe who killed Niall Garve's young son (and Conn's nephew) in retaliation.
[4] Conn played an active role in the fight against Hugh Roe, most notably during the Siege of Donegal in 1601.
It was a lengthy and hard-fought conflict during which Donegal Abbey was wrecked by the explosion of barrels of gunpowder which had been stored there.