I asked a Sinn Féin councillor I know about this and he's just come back to me after speaking to the council (DCC). They told him that if you're doing Shared Ownership, you can't rent a room, because you're effectively renting the property from the council and so renting out a room would be considered subletting, which isn't allowed. However if you have a mortgage from a bank, it depends on what your agreement with the bank says.
Can anyone confirm that Fingal or other councils allow you to rent a room even if you're doing Shared Ownership?