If you talk to most of my fellow micks, they'll whinge and moan about Yanks 'pretending' to be Irish. They'll take offence to it, like Irishness is little more than a fashion accessory to foreigners. They'll say that such Yanks are annoying and will scold them online for saying "I'm Irish" when they're several generations deep into being American.
Fuck all that noise. If someone wants to be associated with my wee island (and it is mine), I get a warm feeling in my chest like an internal hug. I don't care if the person has never had a single ancestor so much as set foot on Ireland, if they nevertheless wanna be Irish I'm thrilled to hear it. It makes me less self-conscious about my shite accent, it makes me feel like I could have friends in other countries before I even meet them. It's like a cheat code whereby I'm granted, up front and gratis, 30% of the required 'social ingratiation' mileage you usually need to put in before you start getting somewhere with a new friendship. There is literally no downside to it that I can see, and certainly no downside that has ever presented itself to me. The only way I could feel negatively about it is if I already have a casual hatred of Americans, which I don't.
If you don't believe that the cunty "ur not irish m8" attitude is borne of hatred for Americans, show me the reams of similarly-scolded Australians claiming Irishness. Show me the Iraqi with an Irish grandfather being told to shut the fuck up for claiming Irishness. Show me the hoards of Englishmen, even, being told the same. I have only ever seen or heard it directed at Americans.
The Australia example is much more powerful because it's one of the two places Irish people tend to go when they leave the British Isles, the other being the USA. We have been going to Oz in droves - voluntarily or not - for about as long as the USA has existed, so we claim a huge portion of the populace. The USA is still a brand-new country, in the grand scheme of history, so there're legit reasons for people to still feel like their families' previous countries are the more defining ones for their identity. There's nothing wrong with that. Unless you're American, apparently.
In closing, let me talk directly to my fellow spud-botherers: please shut the fuck up. I assure you, between the scolding Irishman and the Irish-loving Yank who never shuts up about it, you are the annoying cunt in this scenario. You make Irish people sound like völkisch wankers obsessed with the purity of their genes and the ethnic make-up of their country. Spend less time gatekeeping national identity and more time on avoiding heart disease, ya greasy hoors.
It's just that I find the heritage fetishism some Americans spew is a bit weird and overdramatized.
One of the problems in the US is a lack of temporal density. It hasn’t been here long enough to really have all that history. The people search for an identity elsewhere.
America is a nation of immigrants, from everywhere. In most countries, most people can trace the majority of their heritage to the region they were born in, back for hundreds of years.
I'm an American, I have one grandparent who I can trace back to the colonies. The other three lead to different countries within 150 years. Both of my paternal grandparents were immigrants, my wife is an immigrant. This isn't uncommon at all in America, and the "heritage fetishism" just strikes me as an acknowledgement of this. People want to learn about their roots.
The Irish stuff specifically is a bit weird though.
My parents referred to my siblings and I as “ketchup kids” because we were a little bit of everything mixed together at some point, so even being as fully American melting pot as possible, there’s a heritage identity there. It’s just something we tend to be conscious of.
My step dad is Irish and was adopted here when he was 4, but that’s the only person in my family with a direct single ethnic lineage for at least two generations.
There's definitely some heritage fetishism in the US especially around various holidays Americans associate with different cultures, but like OP said, the US is a relatively new and young nation in the grand scheme nations. Our national mythology is all about the different immigrants that came here and left their mark on the culture. As immigrants came over most of us had to hold onto the old country ties and connections, it's why so many cities have a Chinatown or a Little Italy, and these places took traditions we had from the home country and adapted them to what was available.
We all have a general implicit understanding that were all Americans, but the lineage we trace our families back to is why we say "I'm (Heritage XYZ)"
As for a lot of the really weird stuff especially around holidays (Cinco de Mayo, St. Patrick's day, etc) that's mainly just an excuse to get drunk and party and is definitely pushed on by the entertainment and hospitality industries to make more money.
I'm German (via my great-great-great-grandmother) so things like that don't bother me
The what's your ancestry conversations white people have with each other are annoying. You ever hear a white person try to have this conversation with an African-American? I have. Yeah it was extremely cringe. Then there's the claiming to be such percentage native. Somehow it's always Cherokee. This shit kills me.