I received about €500 of dividend income from a UK listed company in 2020 which I forgot to declare. Firstly, should I still declare it at this stage? And does anyone know what penalties might apply given the lateness of it?
Yes. You should be able to amend it on ROS. Basically follow the same steps as if you were filing an original return. There are a few boxes to tick on the first page of the return and you'll obviously need to enter the dividend income and rerun the self-assessment procedure but if you miss anything the program will highlight the omitted items so it's hard to go wrong.