1. you have to have owned the car for 6 months.
2. you cannot sell for 12 months after you come in.
3. you need proof of living outside ireland bills etc.
To add to Frank's comments you need to be properly transferring your residence from UK to Ireland to avail of the VRT exemption, so you need evidence such as a job offer, property purchase/rental in Ireland too. Plus, if you've already used the exemption in the last 5 years, then fuggedaboudit.
Unless you need three cars over here you may be as well selling one of them in the UK, given the state of the car market in Ireland.