@Eyerollcentral
I am Irish. The cringe comes from the mixture of pity and second hand embarrassment one feels from witnessing someone trying too hard.
This is not something you should accept as innate, unchangeable, or immutable about yourself - it's not any of that. It's a perceptual problem arising from a specific cultural milieu.
Being Irish (from NI) doesn't mean you don't suffer from a peculiarly British problem based in class boundary policing, and NI comes with its own set of problems based on group boundaries and very rigid policing thereof.
It’s nothing to do with ‘staying in your lane’ I am personally very much not someone who stays in their class lane.
You have said several things on this thread that suggest otherwise.
You have claimed to use the Hiberno English of your place of origin despite living elsewhere. You have used phrases indicating a heightened radar for 'pretending to be someone you're not', and 'artifice', etc. You recounted a story of 1 in 100 children who picked up an American accent in the US, which you still call 'attention seeking'. This is all 'stay in your lane' preoccupation whether you recognise it as such or not. I think you do have a strong sense of group identification and an unwillingness to stray out of your particular lane.
People can usually identify quite easily where a person is trying to appear to be cooler or other when they aren’t. That’s a perennial source of humour as all humans can relate to someone trying to pretend to be something they aren’t. The tension there builds the humour.
Or maybe they're not trying to amuse or entertain you, and the way they speak is really no business of yours, certainly not something to get your knickers in a twist about?
The discomfort and the fixation with 'artifice' or people allegedly 'pretending to be something they aren't' is all about boundaries - policing them, identifying breaches, judging and punishing those who stray across them. Staying in your own lane can apply to class, to nationality, to tribal identity. Americans are the ultimate unplaceable elements in the British world. You can't tell from looking at most of them or listening to them how much they make or what their job is. They are casual, they don't speak or observe the British code - hence the hackles, because having a place and knowing that place is very important in the hierarchical British system. The British people who use American slang are guilty of thumbing their nose at the system so many of their fellow citizens are heavily invested in. It's the ultimate British social treason to simply stop playing the game, stop speaking the code.
"‘Was the 1 out of 100 children who retained the American accent directly challenged about it all those decades ago? Was he the subject of conversations behind his back? How very sad, if so...’"
...they were ridiculed to their face, usually most of all by their own family. Because the 99/100 other children managed to come home speaking the same as they did when they left. There’s always one isn’t there though 🤷♀️ Attention seeking. It is incorrect to say retained the accent, that would imply they had the accent to begin with. They didn’t. They put it on on their return in an affected attempt to, guess what?!, get more attention!!!!
There’s always one isn’t there though
They put it on on their return in an affected attempt to, guess what?!, get more attention!!!! 
God, that's even worse than I imagined at first.
The fact that you seem to have absolutely no understanding of this child and in fact join in the judgement of people who, frankly, bullied him explains a lot. NI has its own dynamic of course - deeply ingrained tribalism. Woe betide the person who steps out of their lane there.