The Irish were the largest ethnic group ever to enter the United States. Today, there are over 43 million Americans that claim Irish descent. Although the nineteenth century Irish Potato famine is the most well known cause of Irish immigration, it can also be linked to the Protestant and British mistreatment and oppression of Ireland's countrymen, specifically of those from the Catholic faith. Irish immigration into America eventually led to 180,000 armed and fierce fighting soldiers being placed onto Civil War Battlefields, of which 144,000 served on the side of the Union. This thesis is intended to demonstrate through documentary evidence that the actions, prior to, during, and after the American Civil War of nineteenth century Irish immigrants saved America's Union. Specifically, I uncover the historical significance of the collective conditions of Irish oppression and failed Irish white potato crops to demonstrate the relationship these conditions played in Union tactical success during the Civil War. These two forces were vital to the salvation of the Union.