When I lived in Copenhagen a few years back the commuters numbered in the 10000's. I can imagine that number has gone down a lot recently.
See: https://www.poliisi.fi/identity_card/use_of_the_identity_car...
This is how travel works right now between the UK and Ireland, hence why it's such a big pain point in regards to Brexit.
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.
But yes, I kinda expected a regular ID would be enough, just as it has been in the past.
Does your regular ID state your citizenship? Good, then you can travel. If it doesn't, you aren't proving that you are a citizen with the right to travel freely in schengen.
No amount of explanation helped. I did have my passport with me and I showed it to this lady. Rest of the trip I used only the ID card.
Common theme seems to be, at least in Finland, that dealing with public agencies and authorities is just fine, but you can get to a trouble with private companies.