How long have you owned the apartment? There's a possible Stamp Duty clawback.
You inform Revenue immediately so that they can stop the TRS. Then you declare the rental income by self-assessment later. You can claim the interest on your mortgage against your rent, as well as your life assurance, property insurance, maintenance of the property etc. Revenue has plenty of useful information on their www.revenue.ie website. Do a search for "rental income".
I own the apt. 2 1/2 years and bought it new as a first time buyer so did not pay any stamp duty. So should I assume there would be no issue with a stamp duty clawback?
I will do also a search on the revenue website as you had suggested.