I'm not sure I understand the premise of the question. Such a policy has never, to my knowledge, been implemented.

Irish is considered a mandatory course in school, and to get into university, it's one of the three pre requisites that one has to take when leaving Secondary school. The other two are English (which in American is "Literature", it's assumed you have a mastery of the English language) and mathematics.

There are two levels of Irish one can take. Standard (lower) level is the language itself, grammar, basic vocabulary, and requires being able to hold a very simple conversation. (I had a gimme in my oral exam as I had a cast on my arm so it was obvious what the discussion would be about). You might have to read an Irish book, folks my age will still shudder at the name "Peig", thankfully now gone some decades from the curriculum. Higher level is closer to true fluency, most people don't take it. I can't imagine getting a good education in other subjects at lower level Irish.

If you are a monophone, for example if you were raised abroad and only moved to Ireland late in your education, one could apply for an exemption for the Irish requirement for college, but basic Irish was simple enough that I took it in my case anyway and got a pass. That scene in The Guard where the FBI agent is getting stonewalling, I actually generally understood.

It is also an option to be taught fully in Irish, my cousins went that route with their kids. They do, however, speak Irish at home, so in their case it actually is a first language so the arguments about failing in practical education do not apply. (For the record, they live in Dublin, not the Gaeltacht, where Irish is spoken.

I am not aware of any serious attempts in the last century to make education in all subjects in Irish mandatory, the system currently in place does a reasonable job of keeping the language alive in the places it is routinely spoken, and the basics of it known to the population at large.