Parishes are generally not the same as the An Post areas. Largely similar, of course, but there are differences.
In the case of Killester parish vs. Artane street address, I’d say it’s more likely to be Artane. Gospel according to An Post, as it were.
Concrete example - Balheary Church is in Balheary as far as street adresses go. However, it’s Donabate parish, despite being about six miles outside Donabate and the other side of the motorway.
There’s no real (legal) boundaries on areas, they’re just amorphous blobs decided by people on an ad-hoc basis so there’s no right or wrong name.
My estate is between Santry, Coolock and Clonshaugh and on the point where two roads meet, so An Post instructed us just to use , Dublin 17 and to omit both the area and the road name.