Interest relief that has been denied for a particular chargeable period
because a tenancy was not registered by the return filing date for that
period can subsequently be restored if the landlord avails of the late
registration facility, subject to the usual 4 year time limit on claims for
repayment of tax.
So my reading of this is that you can indeed back date it and the interest deduction will be reinstated. Revenue dont appear to be that interested in disallowing mortgage interest generally. They are much more interested in using the PRTB to find those people who are not declaring the rental income at all by communicating with the PRTB and also the Department of Social Protection where the tennents are claiming rent allowance.
capnhand