I could do a seriously long St. Patricks Day post, but I’m busy and lazy all at the same time, so I’ll direct you to this blog, who has already done the work.
Most of the time, white people consider celebrations of European heritage to be racist unless they omit large swathes of the 16th through 20th centuries. But since the Irish never engaged in colonialism and were actually oppressed it is considered acceptable and encouraged to celebrate their ancestry. For this reason, 100% of white people are proud to claim that they are somewhat Irish.
A big part of St. Patrick’s Day is having white people feel particularly upset at the oppression of their ancestors that has in no way trickled down to them. If you find yourself talking with a white person who tells you about how their great grandfather was oppressed by both the English and the Americans, it is strongly recommended that you lend a sympathetic ear and shake your head in disbelief. It is never considered acceptable to say: “but you’re white now, so what’s the problem?”