At times he had a strained relationship with his father, and offered his support to Turlough's rival Hugh O'Neill, Earl of Tyrone.
He offered important assistance to the English commander at Derry, Sir Henry Docwra and took part in a force that captured the strategic settlement of Dunnalong.
He was also present at the Battle of Lifford, where an important victory was won over the rebel leader Red Hugh O'Donnell.
This would have brought him control over vast swathes of central Ulster, but he died suddenly in late October 1600.
His death is generally attributed to heavy drinking at his wedding day celebrations, but he may have been a victim of a fever raging through the army camp.