Looking only at foreign-born soldiers in the Army of Northern Virginia in mid-1863, I have thus far identified over 350 individuals out of an estimated 3,500 serving at the time, which break down by birth country as follows:

Ireland-58.4%
German States-16.5%
England-8.0%
Canada-3.7%
France-3.4%
Scotland-2.0%
Italy-1.1%
Mexico-1.1%
Switzerland-1.1%
All others- 4.7%

In the 1840s, famine caused by the potato blight drove the Irish to emigrate, while it was social revolution that drove the Germans and other Europeans to our shores. Otherwise we would have little reason to celebrate St. Patrick's Day or Oktoberfest. In the Union Army of the Potomac, foreign-born soldiers comprised upwards of 25 percent of the total number. If one concludes that foreign-born manpower turned the tide for the North, Irishmen can take most of the credit; they made up the bulk of the immigrant soldiers in both armies.