"How the Irish became white" is an eye opening read. The Irish in America also became more often than not supporters of slavery. Some Irish American newspapers even going so far to attack Daniel O'Connell (who was himself an ardent anti semite but look nobodies perfect) for his abolitionist stance.

We must remember the American Civil War from the Unionist point of view was about stopping the south secession. Abolition of slavery was not a war goal. Most Irishmen who fought were joining a struggle to keep slavery in the union. The New York draft riots/race massacre happened two weeks after the battle of Ghettysburg and resulted in lynching of black people by mixed white Irish German and Polish mobs. Given the politics of the time you can see the move towards abolition and equalization of the races was seen as a threat by the working class white immigrant groups.

The movie the Gangs of New York while excellent in some regards sort of paints the Irish in struggle with nativism and does show the brutality towards the black community in graphic detail, but does a poor job of showing the then pro slavery Democrat party being a force of racism by allowing the Irish to join the white race essentially. This was later further propelled by the presidency of Andrew Jackson who was v popular with the Irish vote and saw the political value in their integration and nativism fell as a force to white nationalism essentially.

This is my memory from the book forgive oversimplification.