start.
You and your brother bought a property. You resided in the property for 3 years. Since then the property has been rented out. The rent a room relief will cover your brothers exposure to tax but have you dealt with the fact a property you own a share in a property that has been rented out?
Your share of the rent has paid down your share of the mortgage. Is this what you refer to as the amount your brother is paying you for the amount overpaid on the mortgage?
You currently owe (your 1/2 share) €90,000 on the house worth €82,500. Your brother is paying this down to €66,000.
He is paying down your debt by €24,000.
I think that you may have an income tax issue but €24,000 would be covered by the class B threshold unless you have used your threshold previously.