"Is the party good?"
Translation:An bhfuil an chóisir go maith?
Let me call my Irish book (Doyle, Gussmann, An Ghaeilge) (this is translation from Polish):
Particle go is semantically empty and it is decided by syntax considerations whether it appears or not. It is needed for example in interrogative pronoun cad 'na thaobh - why? In this way any verb that appears after cad 'na thaobh go must eclipse first consonant or add n to vowel, e.g.:
Cad 'na thaobh go bhfuil siad ag obair? - Why are they working?
Cad 'na thaobh go n-ólann sé bainne ar maidin? - Why is he drinking milk in the morning?
In sentences with negation go is replaced with ná which doesn't eclipse next word but adds h to vowel-first word - ná fuil sé and ná hitheann sé.
So it is the same thing with maith - as galaxyrocker said - it needs go first in some cases. For your question it seems that maith is an adjective and go maith is an adverb. Which changes completely the meaning.
Táim go maith - I am good/ I feel good.
Táim duine maith - I am good person.
I hope that helps you a bit.
I think of 'go' preceding an adjective in Irish like adding 'ly' to the end of an adjective in English. It doesn't always work, indeed it doesn't really work in this case, but it helps when deciding if I should use 'go' or not. eg 'díreach' - straight. 'go díreach' - directly. 'Mall' - slow. 'Go mall' - slowly etc.
Whether a party is good or not is a subjective determination — a question of state rather than of characteristic — so An bhfuil … ? is the appropriate way to ask the question.
“Is it a good party?” and “Is the party good?”are both "subjective determinations" even though “Is it a good party?”/An cóisir maith é? uses the copula, but "Is the party good?"/An bhfuil an chóisir go maith? uses bí.
The NEID recognizes it as a possible translation of “fun occasion”, but cóisir seems to be preferred. Páirtí seems to be a direct borrowing from English, since it’s preferred for “political organization” and “side in legal dispute”, and cóisir isn’t usable for either of those meanings.