Under S.216A(4) "Rent a room relief" it states "The provisions of the Income Tax Acts relating to the making of returns shall apply as if this section had not been enacted." This means that if you are a chargeable person you must file a return. Under S.959B(1)(a) of the TCA 1997 FA2016 the income thresholds has been increased from €3,174 to €5,000. i.e. pre 2017 €3,174. This threshold will include your Rent a room income "as if the act S216A had not been enacted". If you do not fall into this chargeable person category there is no legal requirement to file a return unless the revenue ask you to submit same.
However in the Revenue guidance 7.1.32 "Rent a room relief" Paragraph 6.1 states "Although the relief applies automatically, an individual who is required to submit an annual return of income must, nevertheless, enter the amount of exempt rental income on the return (in the ‘Exempt Income’ section of the form). The usual return of income is the Form 11. Revenue may also require certain individuals who are taxed under the PAYE system and who also have non-PAYE income to submit an annual return of income (Form 12)(i.e. chargeable persons under S.959B). Returns of income must be submitted by the return.... before the deadline". I have in the past seen the revenue issue a number of form 12's to people availing of rent a room relief.