For a while the Irish in America weren't considered white. But as the black numbers started to swell and blacks began to move out of the South into other regions, whites needed to bring groups like the Irish and Italians into the 'white' club to keep their numbers up.
The thing is, in America your proximity to whiteness is how you're perceived to treat blacks and your group's mentality towards them. So in order to prove they were down, Irish got a reputation for being hell on blacks, especially Irish police officers.
Now don't get me wrong, the other whites are racist as well...but the Irish had to go extra hard for a while to prove they were really down for the white team. It's like joining a gang, the first thing they have you do is go put in work on the enemies. They have to make sure you're really with their modus operandi and in America the Irish as a group fell in line with the white supremacist program.