Michael,
Over across the river in St. Louis proper the Irish count was about 30%. I would think that it would be the same in the "Metro East" area as well. The largest population in that area were the Germans. That is still true west of St. Louis.
My great grandfather was one of those foriegn born but he had been here about 14 years by that time so I don't see him as "just off the boat".
I remember someone pointing out that once secession started many of the best officers in the U S Army resigned and went south to join the Confederate forces leaving only lower ranking Officers to fill the slots in the Northern army. If they were fresh off the boat that would surprise me in how well they did during the war.
In terms of Chicago there were many Irish but I think that the largest groups were the Germans and the Polish. Could be mistaken since I don't have stats to back that up, but I think I am pretty close.
FC