Hi,
We had an AIB mortgage a few years ago (moved and switched lenders since). They insisted we opened a current account for the mortgage repayments. We didn't have any current account at the time (just cashsave accounts) and I had always banked with them so just opened an AIB current account. The impression I got was that they just needed any current account (obviously its in their interest to have you open an AIB one of course) for the simple reason that if there were not enough funds in the account to meet a repayment a current account would be allowed to go into the red so you wouldn't default on a repayment. However, if its in the T&Cs in relation to the cash back, you may have no choice! Maybe just open one, get the loot, close the account and set up a new DD from your own account for the mortgage repayments!