VRT

When my English flatmate emigrated to Ireland he brought his car with him and becuase he's owned it for more than 6 months in England he didn't have to pay for VRT, just the usual car tax, irish insurance and NCT. My brother did something similar when moving from Edinburgh to Dublin, he bought the car in Scotland while he was living over there a year ago and had no problems inporting it then. It's all on the website though, I wouldn't lie about it, you'll only void your insurance and create a load of hassle. If you buy it in the North, register it up there and then after 6 months or so you should be able to bring it down here. You'll have to get the NCT, car tax and all the rest down here though.

I only know two people who did this - one actually did live in the North with house and all, but worked 5 days a week in Dublin and stayed in a b&b. He was up and down for 5 years and never got stopped.
The other guy was an Ozzie consultant who brought his Land Rover over from the UK when he transferred to Dublin - he just didn't give a toss, he said if anyone tried to hassle him he'd go back to Oz or the UK.
and he did too!