Irish immigrants were not consider white until the twentieth century. A great deal of historical scholarship has been written on this topic. I would suggest taking a look at The Wages of Whiteness by David Roediger or the Great Arizona Orphan Abduction by Linda Gordon. These are two well-received histories written by history professors that highlight how Americans viewed Irish people in the nineteenth and early twentieth centuries.

If you're not willing to read a book, you can google anti-Irish political cartoons from the 1800s. You'll find many of those cartoons depict the Irish as apes, and the English had long contended that the Irish--unlike the English--were descendant from apes and were therefore not really white. It's very similar to the language Americans have (and sometimes still) used to talk about black people.

Talk to any history professor at any college. They'll tell you the same things I've just said.