Being an Irish American myself, I'm curious why they were treated so poorly. It doesn't make sense that they weren't considered white by the rest of the population. How was their life different from other ethnic groups in America at the time?
Many white Americans directed animosity toward the Irish because they were Catholic, poor, and immigrants. The book you might find compelling is Noel Ignatiev's How the Irish Became White. It also shows how race was used (and continues to be used) as a means to police American populations under the pretext of a "white" identity that actually functions in much more nuanced ways than simply refering to skin color.
Hi, this subject used to come up a lot in the old days of this sub; here are a old couple of threads that may still be of interest: