Envy is a terrible sin, Damien. Don't worry, you're not a lesser person just because you only have a 2.5 V6, not a proper 3l one
But back to the original question, if you get the car taxed in the UK, don't forget to claim it back once you have your Irish plates.
Insurance is very important, my insurance company had no problem to insure the UK reg car for a month (I had to get mine fixed before I could take it to the VRT office). I was thinking about using the UK reg for a few months to lower the VRT, but I'm very happy I didn't, as somebody crashed into the back of my stationary car only a few weeks after I VRT'd my car. Surely, there would have been some complications and unpleasant questions asked if I'd still had UK plates on.
Finally, make sure you check what the "experts" write on the form in the VRT office. I had to tell the "expert" who was _copying_ everything from the UK V5 form that Alfa is NOT spelled with "ph".
Good luck, and have fun driving your new car home.
On a sidenote, IIRC Keyser has lots of information about importing cars to Ireland, and I'm sure he meant well when he was suggesting to use the PM feature. There are some information not freely available for the public (like checking for the number of previous owners, etc), but we're one big family to help each other. Whichever way we (legally) can. Just my 2 cents, but I think turning off the PM feature - even temporarily - was a bit harsh.