Yeah you have file a Form 11 using ROS. If you're not registered on ROS then you'll have to start the process now. The return will be late though so 5% surcharge will apply.
It means someone in Revenue will have flagged you as a chargeable person. If your additional income is not regular you can ask them to take you off it. But in may be just easier to register for ROS and submit a Form 11. If your income is straightforward then it's a straightforward process. At least you know about it now -- I spent years not being able to use the Revenue site before I found out what "registered for Income Tax meant".
I have had this issue a few times. Ring Revenue today and say that you tried to file a form 12 last night in time for the deadline but it is saying you are registered for Income tax. That you don't know why as you filed form 12 last year. Ask since you did try and file on time could they waive the surcharge. Worst case they say no. Form 11 is little harder to file but they should take you though how to do it