We use "Is" (rhymes with hiss), rather than tá, in statements of definition and identity.

"Is" is considered a copula because it links together two notions.... Yet I feel like "tá" also links together two notions, does it not?

Is your question about the grammar terminology?

If so, it’s because that’s the traditional naming in Irish grammars, there’s really nothing more to it. It came from Old Irish where at·tá could be used on its own to state existentials (in glosses you’ll find stuff like at·tá Día ‘there is a God’), giving “substance” to things I guess, so it was called “the substantive verb”, while the other ‘be’ construct was called ‘the copula’.

From general linguistic POV both are copular constructs (and one is clearly a verb, the other is rather a predicating enclitic, which turns whatever it’s attached to into a sentential predicate…).

But then, also, the evolutionary biologist Eörs Szathmáry is sometimes being quoted saying that linguists “would rather share each other’s toothbrush than each other’s terminology” and there’s some truth to it. Outside of traditions of specific languages or linguistic frameworks (and often enough inside of those too) the terminology is fairly ill-defined.

EDIT: in modern Irish the usage is a bit different though – you rather need some predicate today, tá Dia ann (though there are exceptions to that even in Modern Irish).