On the surface Kilrain is an Irish immigrant fighting for the Union Army, however I think your question can be better answered by digging deeper into the 'why' of Kilrain's fighting. Kilrain is fighting the AMERICAN CIVIL WAR for his right to be 'treated as he should be treated'. He is not fighting for a side be it north or south, he is fighting for justice...in essence human justice. What is ironic about The Killer Angels is how the insight of one oppressed group (in this case the Irish in America) gains the wisdom that a nation could not seem to find. I have included the 'youtube' site of Kilrain's monologue from the film 'Gettysburg'. As you read the novel feel it, then watch a most powerful clip from the film. It will reinforce your understanding of who (the character type) of Kilrain was and what 'he' was fighting for. Remember, The Killer Angels is a novel and Gettysburg is a hollywood film, however to some extent both convey an intimate perspective, a kind of 'mindset' of Civil War 'history.'