Can you enter Sweden on a UK passport?

No, unless they fly in to Scotland from outside the UK. British and Irish citizens (but not British "nationals" from Hong Kong or other places overseas) can fly within the British Isles with a...

Yes.

Éire is the Irish language name for Ireland. Officially, citizens of the UK or Ireland don't need a passport to enter Ireland from the UK, but all other nationalities do. But you need some form of...

In theory, you don't need a passport if you are Italian but check with the British Embassy in your own country because the rules are complicated.

no - if your are a citizen of Ireland or the UK. Citizens of all other countries may be asked for their passports.