Research has found that children who're raised as bilingual find it considerably easier to then go on to learn other languages. e.g. my cousin's bilingual (Spanish/English) children are likely to find it far easier to learn, say French or German, than children who've only ever spoken English.
So children attending Irish-speaking schools (but who also speak English) will find it easier to learn French than children in schools where only English is used. Similarly, children who attend Welsh-speaking schools in North Wales find it easier to learn French than, say, children in London do.
So, far from being 'disadvantaged', children in Irish-speaking schools have an advantage over those who only ever get to speak English.