When VRTing the car, the address on the proof of address you provide is the one that determines the reg. I just did this recently. Gave a Cork proof of address (my home home) and then once I got the log book and went to tax it, I changed the address to my current one. Got my C reg number on the day in the VRT centre.
edit. tl;dr yes sticking Dublin on it before he dealer registers it should work. Not sure what sort of proof they'll need from a dealer.