Glenn,
According to
this document on the RTA website, plates can be exchanged between cars registered in the same name, by members of the same family, or in other special circumstances.
It then goes on to suggest you call them 131 758 or visit an RTA office.
I think they're making it up as they go along...
I have heard of elaborate arrangements to trade these sorts of plates, that involve selling the whole car to which they're attached, the "new owner" then transferring the plates and getting a regular set for the car, then selling the whole car back to the original owner. Whether that was entirely necessary I couldn't tell you....
cheers
RET