All EU accession states are legally bound to join Schengen, with the exception of Ireland, which during the negotiations for the Treaty of Amsterdam received (along with the UK) an opt-out. No other nations have this opt-out.
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.