To be honest, I'm not convinced that the reason why people have no interest in it is because they had to learn it at school, and it was taught badly.
I'm starting to suspect that this is just the handy excuse. After all, if you wanted to learn something, even as an adult, it's now easier than ever to do so. Just a quick google search reveals a score of free online courses teaching Irish.
Wouldn't it be more honest to just admit that most Irish people, while having a sort of nostalgic attachment to the idea of the language, just couldn't be bothered to spend 5 minutes a day actually learning it, nevermind speaking it?
Then we could maybe try and see if we can find the root cause for this complete disinterest. And I do not believe that can be found in the schools. Bad teaching may well be a contributing, reinforcing factor, but it does seem unlikely that this should have managed to put an entire country off the language.
Make Irish practical and fun and and you'll get kids wantibg to do it. You'll have it booming in less than one gemeration.
Make it awkward, impractical, needlessly base exams and career possibilities on it or use it to be a condescending dick to a nation of teenagers and you'll them off. Of course they won't be put off permanently, as you say; but why would people want to put them off in the first place? For political reasons? Fear? To teach them 'life isn't fair'?
The revivalists are their own worst enemy in this regard.