Agree with SwordsMan, but just to expand the explanation a bit.
AIB gives you sanction to sell your home in negative equity.
This will leave you with a shortfall which is payable over a fixed term i.e. 7 years in your case.
If you do not buy another house, you repay the shortfall over the 7 years. This is fair enough as it's an unsecured loan.
If you buy another house, they give you a mortgage for the new house + a lump to clear the shortfall immediately. So in effect, you are paying the negative equity off over the full term of the new mortgage i.e. 25 years.
It is a bit different for a tracker, but I assume you have an SVR loan.