there could be problems alright!!!
you have to declare/return your rental income received should you wish to avail of rent a room relief notwithstanding the fact there is no tax liability
the legislation dealing with this relief treats the rent received as being nil provided its below the threshold of €7,620, therefore it would need to declared in the first place
secondly, its also worth noting that rent received above the threshold is taxable in full, you cannot just tax the excess over €7,620