I don't think you have much defence, to be honest -
Quote from Revenue.ie;
A qualifying mortgage for the purpose of interest relief is a secured loan, used to purchase, repair, develop or improve your sole or main residence, situated in the State. You can claim tax relief in respect of the interest paid on this loan or mortgage.
Rent Relief for Private Rented Accommodation;
This relief applies to tenants who pay rent for certain private rented accommodation used as their sole or main residence.
Subsection on Mortgage Interest Relief;
If at any point you have a mortgage and are in receipt of Tax Relief at Source you should advise your local Revenue Office immediately as you may no longer be entitled to Rent Relief and your Tax Credit Certificate might need to be adjusted.
So it sounds to me like you can't claim both. As far as "special circumstances" goes, that sounds like something taken from an Iain Banks novel. Unless you have it in writing of course. Even then, Revenue famously won't stand by any advice you get from them. They'll just say you should have gotten professional advice from an advisor.
However it sounds to me like you can claim one OR the other, so do up your taxes and make sure that they are charging you the least amount possible - probably a refund on the rent tax credit.