I dont think you will get the tax benefit on rental profits (invested in a pension) if you dont have Schedule D or E income.
Also, paying back the mortgage will not change you potential CGT charge.
With interest rates low and bonds rates high (see Bord Gais today in excess of 5% yield) I would invest the money in something like this (a lot of the bank also have regular saver products with high interest rates).
If you took out your mortgage years ago you probably will have a pretty competitive rate therefore investing your money in a safe product may be a better use of funds.