Can you not order from a GB forecourt/car showroom, and then just drive it back to NI, and transfer the servicing etc to somewhere local? NI are still in the UK, and Motability are UK wide?
Apart from Motability probably prohibiting it, there are some problems I could see. You would need to register it to your address and that would block it so you would need a GB address. Even if you did have a friend/relative with an address, if you were buying privately by finance, the owners details for the car would need to be the same as rage credit record address so that rules it out. Even if it were possible, you would have the added cost of ferry transport, overnight stays etc. Kezo link above seems to be the best hope.
The new car you buy on the mainland to bring back to NI would only be GB compliant, we have to have cars over here that are EU compliant.