This is the best document I've found so far on the situation post Brexit for importing from the UK and NI.

There are a couple of interesting points in it. A Customs declaration will be required and, despite the agreement, it looks like a significant proportion of cars will be subject to a Customs Duty of 10%. VAT will be charged on all incoming cars regardless of the VAT status on the export. In order to try and use routing a car through NI to avoid Customs Duty it will require that if the car is coming from the UK first, then it will have to be fully registered in NI, with a reg document issued to an NI business or address, before being imported to ROI. It looks like there probably isn't any noticeable difference in how a car from NI is treated for VAT compared to a similar car from the UK.

https://www.revenue.ie/en/importing-...les/index.aspx

I reckon it will take 6-12 months for importers to figure out what "works" in the new circumstances and for the market prices here to stabilize in terms of price levellling to reflect the changes. There may be an advantage to buying Jaguars rather than BMW's.