I mean, OP replied to my answer and apparently liked it, so I think I got him squared. But here’s a real response:

When the concept of whiteness was invented (yes, invented) it didn’t originally include Irish people, and they did endure abuse and marginalization comparable to what black people have endured and continued to endure. Irish people were worked as near slaves, so they even have a lot of that in common. As you say, I think that if you were Irish in America in the early 19th century, people who already belonged to the White club would have mocked you for your corned beef. We still make fun of Irish people for these things.

But there is a difference. Irish people in modern times got access to whiteness. They were accepted as part of the in-group and no longer marginalized. When this happened, and it took decades to gradually go this direction, the mockery didn’t disappear but, if you were Irish (and, in fact, I am) it would have started to feel less like someone who means you harm and more as friendly teasing, precisely because you have access to the same power as the Germans and the British and so on who already belonged to the club.

Black people don’t have that. Black people are still very much marginalized, still the victims of racism and violence and institutional exclusion. So racism, on top of that, is going to feel a lot more painful.

It’s one thing to be mocked; but to be mocked by someone else who is punching down is much worse.