The same rules under Rent a Room scheme apply to people who own affordable housing. You are permitted to rent out a room tax free as long as the income doesnt exceed €10,000 per annum. Although you do not have to pay tax on this income you will still have to declare it to the revenue as I understand it!
If you're on the Shared Ownership Scheme you may need to get permission from the council before renting out a room. If your mortgage is from a bank you should have no problem.