Yeah Northern Ireland could basically just vote to join Ireland.

Scottland would have a harder time. As much as i support the idea they would have to leave the UK, develop an economy good enough to meet the joining criteria (might need to create their own currency), then get permission to join from all member states, including the ones with seperatist movements which dont want to be seen as lenient towards seperatist causes.

It is possible as they could likely use the Euro in the mid term (but this would cause their borrowing rates to be higher than if they had their own currency, as they have no control over monetary policy) and maybe the countries with seperatist movements wouldnt care as they're leaving the UK. But its a bit of a gamble..

Edit: added clarification about currencies