Irish officially the first language, constitutionally. English is an official language too.

Irish is learned by all in school, but spoken as a first language by only a few. Most don’t retain the school learned Irish.

I imagine it’s similar to the level of speaking in Wales and Scotland. However, I think the UK does not have an official language for any of her countries and English is the lingua franca. however, I could be corrected on that, as I recall being told that northern irlend recognizes English, Irish, scots gael and mandarin.

The title doesn’t say spoken language, only uses possessive language. So, the less spoken, but constitutionally first language would apply.