"An bhfuil sé ag cur báistí fós?"
Translation:Is it still raining?
Yes — without context, fós could be translated as either “still” or “yet” in this sentence.
Unlikely fós implies the situation is the same; since it is raining it is "still" raining whereas nach bhfuil sé ag cur báistí fós? Isn't it raining yet
What's the difference between "ag cur báistí" and "ag cur fearthainne"?
There isn't any difference. In Munster, you might come across ag báisteach or ag fearthainn (without the cur+genitive) and in Ulster and Connacht you might come across ag cur without any modifier.