Not necessarily - it's certainly not how you would normally say "it's getting dark" - "tá sé ag éirí dorcha".
But just as in English, it could be a much more threatening statement, (everybody hide, the darkness is coming!) or it could just be a more prosaic "it's after 8PM, so the darkness is coming", though that sounds a bit weird.
This is completely unrelated to this exercise, but your use of the word éirí reminded me of something I saw tonight.
I was watching “The Order” on Netflix (Season 2, Episode 1), and a new character said the phrase Éirí lag agus básaigh when she was casting a magic spell.
The subject of her spell (a rat) then shriveled up and died.
The series isn’t Irish at all, and I don’t yet know whether the character is Irish...but I found the use of an Irish phrase in the show to be pretty interesting!