Irish came in through New York, Boston, New Orleans and down from Canada during and after the famine. Also Baltimore. Scots-Irish came earlier (1700's) and settled the Appalachians to fight the Indians on the frontier. Many settled in Western PA. The Scots-Irish form what we know as the Hillbillies - and have always had a strong, independent mindset - always suspicious of an overreaching government. The Scots-Irish or "American" have always shown a very strong representation in the U.S. armed forces from the Revolutionary War onward. They also were the first to settle the West.
Backbone of the nation. They know how to fight.