Japan is introducing it too by 2030 so the only outlier will be Ireland among the "western" sphere. And as someone who has tried different ETAs the UK's is least laborious (aside from some problems with the app I'd mentioned in previous posts) and has the least invasive questions compared to its peers. And when I looked up the official FAQ for ETIAS it said that it won't be required for transiting though I don't imagine it'll be the huge difference-maker that the likes of LHR's CEO are whining about.