Scotland

I've seen it suggested, and I think it holds some merit, that there might be those in the EU (and accession must be unanimous I believe) who aren't so positive - Spain may not wish to set a precedent for Catalonia walking away and retaining access to the single market.

The other practical obstacle is that like Ireland, Scotland doesn't have a land border with the EU, but does with UK who are the largest trading partner. Scotland entering the EU would reopen a lot of debates around RoIreland/Northern Ireland, which is neatly divided from the rest of UK by the Irish Sea - wouldn't be popular in UK, nor European capitals. Unlike Eire, Scotland doesn't have the option of becoming a floating tax haven.

Personally, I've come round to the pro-Indy position, but its not a done thing that Scotland would join the EU, quickly or otherwise.