One of the most important turning points in Irish history was the Flight of the Earls. On 14th September 1607, Hugh O’Neill, the Earl of Tyrone, and Rory O’Donnell, the Earl of Tyconnell, along with a small party of their family and followers, boarded a ship at Rathmullan and sailed for Spain. The reason for the earls’ flight is hard to determine. There is some suggestion that O’Neill believed the new Lord Deputy of Ireland, Sir Arthur Chichester was attempting to usurp him. King James had reassured O’Neill in 1606 that his fears were unfounded. There are also suggestions that in fact James was on the verge of arresting the earls for treason but no proof of this has ever been found. Some historians argue that O’Donnell and his kinsman, Maguire, were intent upon joining the Spanish forces in their war in the Netherlands. Whatever their intention at the time, they never returned and this left the way clear for hundreds of thousands of lowland Scottish planters to arrive over the next hundred years and change the character of Ulster forever.
Further info is available on Wikipdeia