I don't know how useful it is to be honest really, unless you want to work for TG4 or the EU there are very few areas of life where speaking Irish is actually going to help you.
I do absolutely think it's a desirable thing though, it's a part of our culture the same way GAA or Traditional music is and for me they're all worth keeping alive and doing so is a worthwhile way to spend your time. Being educated through Irish never hindered me or most other people I knew, if anything the evidence would suggest it has a positive effect on eduction. I know what you mean regarding other languages, but learning further languages is much easier if you're already bilingual (didn't help me though, my attempts to learn both German and French failed miserably). I'm not convinced that foreign languages are all that much more useful either, unless you're going to live abroad (in which case you'll learn the language pretty quickly anyway) there are very few contexts where you're going to end up speaking French/Spanish/Mandarin/whatever, while I do actually end up speaking Irish a fair bit (admittedly all to people who also speak English). It also is genuinely helpful when you're abroad to all be able to speak a language that nobody else in the city can come close to understanding (although I'll admit that learning a languages for the sole purpose of complaining about foreigners is probably unnecessary).
At the end of the day I'd kind of characterize it like learning a musical instrument, both are unlikely to get you anywhere in terms of employment or to do much for your long-term prospects but it doesn't mean they aren't a valuable use of your time.
At the end of the day there's nothing really there to show that teaching kids through Irish in school is doing them any harm (the opposite in fact). At the same time, although I'm not really qualified to comment on how people learn Irish in the rest of the country's schools having never gone to one, it doesn't seem like it's achieving much good but at the same time I think we're better off at least making that link than not bothering.
I do think that it is important to maintain our culture, especially in a country with a history as broken and tragic as Ireland's, and I think there's an element of responsibility to keep it going there as well, I'd make an effort to speak as much Irish as possible and if I ever have kids I'd absolutely send them to a gaekscoil in all likelihood.
Fair enough, it's not really my place to say. And I fully support that there are other reasons than employability for a school subject to be worthwhile, the musical instrument analogy is a good one.
I suppose my distinction is between having the
opportunity to learn the language in school (which I fully support), and the
obligation to. As an example, my cousins are Scottish, to a Scottish mother and English father, lived their whole lives there and don't really identify as English at all. My Uncle and Aunt fought against their obligatory Gaelic lessons, because they insisted they considered it a useless skill and would rather they learned a "useful" language. I agree the definition of "useful" is subjective (and when English is THE business language, is any other language actually much use to most people? maybe not ..) but I do see their point, and I do feel they should have the right to decide that for their children.
I haven't seen (or looked for even) any research on how gaekscoils or their equivalents impact education overall, do you know any (ideally not too dry...)?
Purely anecdotally, a friend of mine's experience does show an obvious negative effect, although how widespread it is I don't know. He spoke English at home but went to a Welsh-language school in Swansea up to GCSEs, did everything in Welsh. When he switched to an English language school for 6th form, he was immediately at a disadvantage - although obviously he spoke English as his mother tongue, there was actually vast amounts of vocabulary that he had never come across and had to relearn from scratch! Even a word like "textbook" he said, he had never heard in English, not to mention whatever subject-specific words he had learnt - imagine doing a geography A Level having never learnt the words for different types of rock at GCSE! For some that's pretty quick to overcome, but he's dyslexic and not naturally good with language, he is adamant it held back his education.
Last point is just an observation on this:
I know what you mean regarding other languages, but learning further languages is much easier if you're already bilingual (didn't help me though, my attempts to learn both German and French failed miserably).
Again, not based on proper research, but I taught EFL for 4 years so on the more reliable side of anecdotal ... my experience was that genuine from-birth bilingual people, Catalans or Moroccans for example, were not noticeably better at learning English. They had some small advantages - for example most people when first learning a language are very confused by the realisation that the grammar of the "new" language is completely different ("I like you" in English as against "je t'aime/I you like" in French for example); if you've never thought about grammar, your own seems like the only possible way of doing it, but if you're bilingual, you're already past that hurdle and much more open-minded to the weird differences between different languages. But overall, I didn't feel they picked up on what I taught quicker than others.
(As a side note, and only because it's interesting, not because it's relevant - people who were good at music, I noticed, often had better accents. Weird)