A very misguided superiority complex. Fair enough, if the OP were to ask what to wear to an English church wedding. But she isn't asking that.
I'm not Irish, but my brother is married to a wonderful, lovely Irishwoman. Our weddings are quite similar to Irish ones, in fact. Ours traditionally lasts three days, although more modern ones are often shorter. They're great fun, yes, there's lots of dancing, drinking, singing, games and entertainment. Lots of food. The wedding is fully catered by the bridal party, none of that 'pay for your drinks' nonsense. People spend a lot, and people gift a lot. And go all out with their clothes; it's very usual to see full-on evening gowns, although it's not a strict requirement. But it's very, very dressy.
Frankly, I'm often absolutely gobsmacked by the dresses/jumpsuits recommended for weddings here. The sorry-looking, drab, floral and dowdy midi nighties from Rixo or similar. They're not 'classy'. They're drab and more suitable for drinks and lunch (and even that, depending on where you're going, if the place isn't very nice) than for a wedding. Stuff like Nobody's Child and similar, I wear those cotton dresses to the beach or a supermarket, not a wedding. And no, you can't 'accessorise' a cotton or linen sack, even full on diamonds won't help with that.
OP's dress still would be a bit too simple for our weddings, but with hair, makeup and jewellery, good shoes, etc, it would be fine. No one would bat an eyelid at its colour, shoulders or think it's 'tarty' or whatever other medieval nonsense. Irish weddings I've been to - same, as was confirmed by multiple actually Irish posters on here.
But no, apparently she needs a 'classy' floral sack. Oh yes, and a fascinator, can't forget those.