Because they pinky promise? It'll be required to produce one like producing a driving license is required, but you don't need to have it with you. It'll be required because pubs/bars etc will require it. It'll not be required like giving a traffic stop breath sample is not required or like giving up your password is not required.

Imagine it was called IrishID or similar, help you to have empathy for how half of NI residents might feel about Brit-anything?

Huh? Everyone carries their phone everywhere and an ID card on an app is carried everywhere. When you stay at a hotel you might have to produce it, go to a doctors appointment,... and so on