I recently listened to two songs, "The Fighting 69th" and "We'll Fight for Uncle Sam" both of which describe Irish heritage regiments/soldiers in the American Civil War. Considering that many Irish immigrated to the United States less that 25 years before the start of the war, what rhetoric was used to inspire them to take up the cause of a country that was new to them? And what where the thoughts of the Irish soldiers themselves?
Whilst I can't confess to know exactly, it could be seen that the USA was an ideal for the Irish, successfully defeating the British and gaining independence. Therefore, many Irish may have wished to defend that, especially considering there was little chance of returning home to Ireland.