American history why did immigrants came to the united states in the 1800s feel discriminated against?

Immigrants like the Irish, Scottish and others were flooding the U.S. the Irish were especially discriminated against because there were so much Irish coming into the U.S. in hopes of food and work, there weren't that much jobs available so lots of Irish weren't even getting what they were looking for in the U.S. in the first place and they were looked down upon. But hey, we have Saint Patrick's day now so jokes on Americans haha.