"It is night."
Translation:Tá an oíche ann.
There's a couple of different things going on here. It is possible to use an implied copula to express this (('s é) (an) oíche atá ann) as some of the earlier commenters suggest. But, as scilling pointed out in the first response, this is a statement of existence, not a classification or identification - the "it" isn't a specific thing that is being equated with "the night", it's just a general placeholder, so it doesn't fit the basic definition of a copula, and bí ... ann is used to express such existence
In Ireland we sometimes use the phrase "the day that's in it" even in English (from an lá atá ann) and bhí an-oíche ann! would be a natural way to say "it was a great night!".
The FGB entry for ann starts off with "There (in existence)" and gives these usage examples:
Tá Dia ann - "God is" ("there is a God")
Tá an saol ann - "the world exists"
Tá lá maith ann - "it is a good day"
Tá an t-earrach ann - "it is spring"
Tá an uair ann - "the time has come"
Bhí fear ann fadó - "there was a man long ago"
Dá mbeinn ann an uair sin - "had I lived then"
Nuair a bhí m’athair ann - "when my father was alive"
I'm afraid that there might not be a simple, obvious reason that "it is night" isn't a copula in English, but that would require a deeper examination of English grammar. For the Irish grammar, bí ... ann is key.