A very interesting situation.
First of all, don't worry about your marriage affecting your wife's tracker mortgage. It won't.
If you are not in a mad hurry to trade up, and it seems as if you are not, then I think you should be more relaxed in your negotiation. Ask AIB if they will give you a discount for paying off the tracker early. Don't confuse it with asking for a new mortgage. If you get the discount, then put the property on the market.
Likewise for UB. If your co-owner is prepared to sell, you should do so if UB will give you a discount.
I don't think that UB will give you a discount. AIB probably won't give you a discount now, but they might at some stage in the future.
If you don't get a discount, stay put and continue to save, until you absolutely have to trade up. Even then, you can let the AIB apartment and rent a new house. As it's a cheap tracker, you will probably find that the rent exceeds the mortgage payment.