There's a difference between a blood heritage and cultural heritage, you can have both, just one or a mix.
For example: my friend was adopted as a toddler from southeast Asia, she did a dna heritage test earlier this year and found that she's 84% Thai with some Malay and Belgian thrown in. She knows very little about her Thai heritage, (and hates spicy food) but that doesn't make her any less Thai than anyone from Thailand.
Idk man, when some americans tell me they are Irish and they just well, they don't know how to have the banter, I really think that a lot of the first generation Nigerian and Polish immigrants that live and work here are way more Irish than they are. That's just my opinion. I don't think race or ethnicity means much, to me I'd just say, yeah your friend is (I assume) an american, culturally american, and going to act in accordance with american social customs. So I just don't get why this ethnicity stuff matters. But that's just my opinion. I feel like America is such a huge place ,and so many people never travel outside of it - I feel like that's the real crux. A lot of people they like to say, oh I'm this but they don't know how it FEELS to be that I guess. They don't know squat.
TL;DR sick of people who don't know directions to the best pub on the keys acting like they are Irish. Most of them are just white americans, completely removed from what it means to be Irish, to live in Ireland.... They don't know any of these things. So why say you are Irish? I just don't get it at all lol