Causes
The Nine Years' War was caused by the collision between the ambition of the Gaelic Irish chieftain Hugh O'Neill and the advance of the English state in Ireland, from control over the Pale to ruling the whole island. In resisting this advance, O'Neill managed to rally other Irish septs who were dissatisfied with English government and some Catholics who opposed the spread of Protestantism in Ireland.
Read more about this topic: Nine Years' War (Ireland)