By way of an update. I spoke with the RTB today and they confirmed that I do not have to register in this instance unless I want to. However, they said that without registration that I cannot claim relief on the mortgage interest or that my bro in law can claim the renters allowance. She said that this is linked to a RTB registration number.
I have called Revenue to try and get more information as it seems a bit daft that when you are exempt from the RTB registration, that you still need to do it for the tax benefits, but is Ireland... I'll let you know of any developments. Thanks for the responses so far