Renting room to relative

As long as they're not your child or spouse/partner and they're renting a room in your PPR, you can charge them up to €14k in rent a year without having to pay tax on that rental income.

If they are a dependent person who is permanently incapacitated due to disability or is over 65, you may be eligible for the Home Carer Tax Credit.

If they are not dependent on you or don't otherwise meet the above criteria, then there aren't any tax credits you can claim that I'm aware of. Your relative, however, might be accruing CAT liability based on the market value of the room you are letting them use for free. If so, then they'd need to track that liability, and depending on which CAT threshold group that falls into based on your relationship, they might cross the 80% reporting threshold eventually, or even cross the threshold itself and start having to pay CAT on their free rent.