Ag báisteach is the verbal noun of the regular verb báistigh, meaning rain.
Thus, ag báisteach: raining.
In Connacht and Munster, this phrase is often pronounced a' báisteach in naturalistic speech.
The phrase ag cur báistí (literally, 'at the putting of rain'; often pronounced a' cur báistí in Connacht and Munster) has the same actual meaning, and is widely used.
Another common word for rain is fearthainn (ag cur fearthainne / a' cur fearthainne).
In Munster, heavy, pelting rain is called clagar (ag cur clagair / a' cur clagair) and sheeting rain is called clagar na gcos (literally, 'the heavy rain of the feet'; figuratively, 'walking rain').