To sell into the EU you will need an EU registered Responsible Person – they won’t be able to be registered in GB (hence why GB based Responsible Persons will be losing access to CPNP).
About Northern Ireland (NI), whilst this is a UK service, there is more EU alignment in NI. A NI based responsible person (RP) who sells cosmetic products only in NI will only need to register on the EU’s CPNP. If that NI based RP wants to sell in GB, they need to register on the UK Submit Cosmetic Product Notifications (SCPN) service as well. Selling a new cosmetic product in both GB and NI markets would therefore require registration on both services. Obviously you could have a UK established RP in NI who only sells into GB, in which case they would only need to register on the UK’s SCPN