Reminds of the saw doctors song, "Share The Darkness". It contains the lyrics "Tá an dorchadas ag teacht inniu, tar abhaile liom mo ghrá".
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.