There were many ethnic groups that fought in the civil war none as famous, or as respected, as the Irish. 140,000 Irish-born soldiers in the federal armies, 1/3 came from New York. An all Irish brigade was formed in November 1861 that comprised of the 69th, 63rd, and 88th New York regiments which totaled around 4,000 troops. 70 or so would make it back to New York after Gettysburg unscathed, after the decommissioning of the brigade following the New York riots. Irish immigrants joined the union army during the civil war for many reasons such as patriotism both for the US and Ireland, hopes that the US would aid Ireland in its quest for independence or Fenianism, and the hope to be treated as equal.
It is important to look at the jobs readily available for immigrants today as those same manual labor jobs are worked by recent immigrants mainly Hispanics. Many Hispanics join the army today due in large part to the DREAM Act or programs like it that gurantee citizenship after 8 years served in the armed forces. "Between 2001 and 2005 the number of Hispanics enlistd in the Army increased by 26 percent. There are nearly 213,965 Hispanic soldiers currently serving in the US armed forces. In 2010 Latinas made up 19 percent of all new enlistees, and serve more than latino men as Latinas make up 7% of veterans and Latinos make up 6%."
Cardenas, Vanessa, and Sophia Kerby. "The State of Latinos in the United States." Center for American Progress. August 8, 2012. Accessed December 11, 2014. https://www.americanprogress.org/issues/race/report/2012/08/08/11984/the-state-of-latinos-in-the-united-states/.