That’s a very valuable CGT “shield” that would argue in favour of retaining the rental.
Say you sold the property after 20 years ownership, you would only pay CGT on 7/20 of the realised gain (less disposal expenses, etc, less your annual allowance).
As things stand, if you sold the rental and made a 20% AVC, you would end up with (very) approximately €44k pa after tax.
On the other hand, if you kept the rental and made a 20% AVC, you would end up with (very) approximately €32k pa after tax and €18k mortgage payments.
But of course you would still have the rental, which would be a useful source of income in retirement.
I still think, in your shoes, I would sell the rental and simplify your life by paying off the PPR mortgage.
But that’s a fairly fine call.