"Polish is an important language, but it is not more important than Irish."
Translation:Is teanga thábhachtach í an Pholainnis, ach níl sí níos tábhachtaí ná an Ghaeilge.
Well, the adjective is the comparative form of "tábhachtach" (important) and since "tábhachtach" ends with -ach, the ending -aí is added in place of -ach. So "tábhachtach" becomes "níos tábhachtaí" (more important) (the superlative also uses tábhachtaí: is tábhachtaí (most important)). There is a full explanation of this in the notes of the Comparison skill :)
From a prior lesson's notes: The copula is for when you're introducing something or someone, like "Is bean í" (She is a woman) or "Is úll é" (It is an apple). Bí is for when you're describing something or someone, like "Tá sí ard" (She is tall) or "Tá sé blasta" (It is tasty).
Why then is the copula used here since we are "describing" Polish as important?
In the first phrase we aren't describing Polish as important: we are identifying as "an important language". It's grammatically like identifying it as "a language" or even "one of the things to come out of Poland". "An important language" is a noun phrase.