Yet not all nations are in Schengen. If Scotland were allowed to join the EU (despite Spain's veto) I have no doubt there would be an acceptance that it could remain in the CTA with Ireland and the rest of UK
As far as I understand, the EU closed this loophole for new members but it is turning a blind eye for Sweden.
It is fascinating to me that in practice there is this reversal of roles: Sweden is supposed to adopt the Euro and yet has a free-floating currency, and Denmark that has an opt-out deliberately chose to be in ERM-II and has a currency that is pegged to the Euro.