No doubt that is true but it doesn't seem to account for the discrepancy, at least not without acquiring more data. Google's LLM is spitting out ~250K immigrant and first-generation Irish fighting in the Civil War, sitting roughly at 10% of the total. That doesn't account for second-generation and older, so I would be interested to see what social mobility was like for those more-established Irish-Americans: were they languishing in the working class, or did they exit from the middle class to go fight in wars disproportionately?

edit: how significant is the class factor, anyway? Does that model really apply, and how many citizens would be considered middle or upper class; enough to make a large statistical impact?