There was an article in the newspaper a few days ago about the savings that can be made by buying second hand cars in the UK or the North. Although you will have to pay VRT upon arrival in Ireland, the huge price differential more than compensates.
Apparently 60,000 2nd hand cars similarly imported so far this year. Car dealers expect a glut of 2nd hand cars, and a significant price drop in the 2nd hand market in 2007 because of this.
Best savings come on high end cars, Mercs, BMWs etc, (up to Eur 10,000), but almost all models will be cheaper if bought in this way. According to article UK cars are generally of higher spec and better maintained than Irish second hand cars.