Was watching the opening sequence to Gangs of New York, the Irish immigrants coming off the boat, to the enlistment stations, to the returning coffins. I know this was done for dramatic affect, but it got me thinking. I've done quite a bit of research on Irish history, but little on the Irish involvement in the civil war. I know there were some prominent Irish regiments, but how much of a factor was the immigrant enlistment, if anything?
In Union armies, the contribution of immigrants was respectable, and not at all negligible. I've seen stats that put the figure at about 22%-24%. You'll often read about notable units, like the Irish Brigade from New York, the Fighting 69th (infantry). They actually fought against another Irish unit at the Battle of Fredericksburg (under Brig. Gen. Cobb). The Union also had the German Corps (the XI), under Gen. Sigel, which saw some of the toughest fighting throughout the war. It's no secret that Lincoln was a champion of Sigel from the beginning of the war, as the young general was a great recruitment tool and inspired a lot of loyalty in his foreign-born troops.
The Confederacy also enjoyed a level of support from the immigrant community, yet since the disembarkation points of so many European travel routes were predominantly in the north (Boston, New York, and Philadelphia), the north reaped a heartier portion of the immigrant contribution.
[Sources: Bruce Catton, 'Mr. Lincoln's Army'; James McPherson, 'Battle Cry of Freedom']