Depending on the amount of rental income (after expenses are deducted) you may need to submit either a Form11 or a Form12.
If the net rental income is less than €3174 and you also have PAYE income you can submit a Form12.
If the net rental income is greater than €3174 you would have to submit a Form11, regardless of whether you also have PAYE income or not.
Luckily, both of them are only due for filing in October 2016.