Things may have changed, but I used to work for a company with a depot in Belfast. We used to bring vans back to the mainland, re-register them and sell them at auction here because prices in NI were so low.
Re-registering is quite easy, all you need is form V55/5. The problem is that if you buy it in NI, you have to tax, insure and register it here before you can drive it here. If it is between 3 and 4 years old, you will have to MOT it as well. My company had no problem with this as we transported them in the back of a truck, but you will probably need to hire a trailer unless the current owner drives it over to you. There are also many companies who will do it for you at a price.
Cars are much cheaper over there, but you do need to factor in the extra cost of getting it home legally.
Thinking about it, I would think that the cheapest way would be to go to NI, buy the car (or do it online in advance) tax and insure it in your name and then drive it back to the UK. You would have to allow a couple of weeks for this. Then you have six months (I think) to re-register it with mailland plates. Whether you can re-register it while it's still physically in NI, I am not sure.