It is a bit more amusing than that, sort of a Catch 22 situation. In neither Ireland nor the UK, nor for travel between them (except by 'plane) is one required

It is a bit more amusing than that, sort of a Catch 22 situation. In neither Ireland nor the UK, nor for travel between them (except by 'plane) is one required to have or produce ID. What one has to do (if asked, and then a right asserted) is establish one's right to enter the country. The easiest way to do that is via a Passport. One can still travel (by boat) between the two countries w/o a passport, you just have to be prepared (if challenged) to argue and maybe waste some time. Driving a vehicle in the the UK (and I assume in Ireland) is a situation where one can be required to produce ID (even then you do not have to carry it), but not in normal day to day life.