Things you hate people saying

The two-syllable pronunciation of "medicine" (and "regiment") is very old. Yeah, it's a class marker, but that doesn't mean its an affectation.

There are lots of other words that look like they should be three syllables but often get a two-syllable pronunciation. (E.g. "interest". And lets not get started on "colonel".) But they don't have the same link to socioeconomic class.

People using "done" for everything in the past; I done it, he done it, what I done was …, etc
It causes an urge that I have to resist to exclaim "DID!".

I get that it is different in the UK for example, where there are parts of the UK that abbreviate words irregularly and that's fine, it's even endearing sometimes, but in Hiberno English it only exists by affectation, it doesn't exist as a result of any differences in accent as far as I am aware. In general I've only ever heard an Irish person say it when it serves as code language, as you put it very well, a class marker, but in Ireland it would be a very intentional learned thing. Children aren't taught in school to say med-sin, at least that's been my experience.

In defending some really nasty individual people saying

" jimmy has a really tough demanding job at all hours requiring a lot of stamina focus and dedication "

110%!!

No, it's a long-established class marker, both in Ireland and in England. Children pick it up at home; that's largely how class markers work.

If you haven't noticed it, maybe you just don't mix with the right sort of people 😉! [Peregrinus ducks and runs.]

Yes , like “You do you, and we’ll do the rest.”