Thanks a mil', epicaricacy. The UK sibling did report that the DVLA end of things was a nightmare on the way over. The only added complication compared to your scenario is that she won't be in Ireland for the re-registration and taxing. Any idea if that can be done with a letter of consent from her, or if the change of ownership can be effected at the same time as re-registering?
EDIT: On looking at this a bit more closely, I don't think the RF111 will suffice. (Thanks for the tip anyway, at least I now know it exists). The RF111 is really to show change of address (or other car particulars). In my case, the current owner isn't changing address, or moving back to Ireland, and the original log book would be no use because it has a different/wrong name on it. In that case I think the change of ownership might have to happen in the UK, then go through the UK certification of permanent export process, and then put the car through the Irish NCTS importation process. I'll check with the motor tax office and the NCTS and report back.