Note that the Irish were not considered white throughout the 18th and 19th centuries and into the early 20th century. See eg. "How the Irish became white":

https://www.amazon.com/Irish-Became-White-Noel-Ignatiev/dp/0...

There were plenty of "NINA" ("No Irish Need Apply") signs throughout the North, the same way we had Jim Crow laws in the South. Other groups too: Poles and Jews were also not considered "white" during this time period, and then gradually assimilated as the nation's racial animus was focused elsewhere. JFK's election as the nation's first Irish-Catholic president was as significant in 1960 as Barack Obama's election as the first Black president in 2008.

Racism as it exists in America is socially constructed, but tribalism is universal. Interestingly different parts of the U.S. have different racial divides, eg. the black/white divide is not nearly so salient on the West Coast, but there is significantly more anti-Mexican racism and economic classism.