"It is dark."
Translation:Tá sé dorcha.
There are limited circumstances in which is could be used with a predicative adjective; in most cases, though, bí is used rather than is.
Permanent/temporary attribute has no relevance to the use of the copula. You won't get a more permanent state than "dead", which is Tá sé marbh, and being a tourist is typically temporary, but it's still Is turasóir mé.
The difference is that marbh is an adjective (that describes a state that happens to be permanent), and turasóir is a noun.
Doesn't "bí" mean "be"? Can I see an example? Google Translate isn't helping, as usual with Irish.
Yes. Tá is a present indicative conjugation of bí, just as “is” is a present indicative conjugation of “be”, so tá sé dorcha is a proper translation that uses bí.
I’d recommend not using Google Translate for Irish.