It makes a difference in the executive/luxury segment, the car with a D will always sell easier in Dublin than a county reg, in the Dublin City motor trade some counties are less liked than others and prices reflect that, Dublin end user buyers simply dont like country registrations ...generally, CN,DL,LK,SO, KY being a few obvious ones, don't jump on me, it is better to have a county reg that borders Dublin if you want to sell any car on, I'm a KE and have had KE on new 5 series which I sold easily, If you buy a luxury car and stick your county reg such as say MO on it rather than D then good luck with that.
Flip side try and sell a D car in Cork....not as popular as a C reg down them parts.