I believe the translation for the above is "Gaeilge is ea i sin!!"

But how would you say "That WAS Irish!!"??

Would it be "Gaeilge ba ea i sin!!"

That just doesn't feel right to me. Thanks in advance.

*Gaeilge is ea í sin. (í with accent)
*Gaeilge ba ea í sin.
more dialectized: *Gaolainn ab ea í sin.

But I doubt they are correct.
Because: There are some idioms with indefinite Gaeilge , mostly with the meaning "(in) the medium of Irish"
They are used a lot ("leabhar Gaeilge", "labhair as Gaeilge", "tá Gaeilge agam", etc.)
But outside of these and esp. when used as a real noun, Gaeilge is usually made definite with an article: an Ghaeilge.

So:
Is í sin an Ghaeilge = That is Irish. or: Sin í an Ghaeilge = That is Irish.
Ba í sin an Ghaeilge = That was Irish. or: B'in í an Ghaeilge = That was Irish.

Oh yeah, because "an Ghaeilge" is definite, the copula rules about identification vs classification apply, so it would come at the end. Makes sense.

I'd definitely say "Ba Ghaeilg é" or "Gaeilg a bhí ann", because when you say "that was Irish", you don't mean "that was THE Irish language" but "that was (some) Irish". It's hard to understand, maybe, to English speakers who normally don't use the article before languages names. In French it's clearer and closer to Irish: "c'était de l'irlandais" (with the partitive article, which has no Irish equivalent, that's why I wouldn't use any article before "Gaeilg" in this sentence).

_________________
Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

With spoken Irish, we just say: Sin Gaeilge!

Hmm, alright.
In German it is actually the same: "Das ist Irisch", gan alt ar bith. Though there's more often an article in Irish with language names than in German (I'd think) but probably not here. And btw: My attempt of an explanation ("(in) the medium of Irish") perfectly fits in here: "That is in the medium of Irish"

So, okay,
Sin Gaeilge (present)
B'in Gaeilge (past)