If we can't figure out the Irish border, what happens if Scotland join the EU?

[northernladuk]

We just add another 16 feet to Hadrian's wall. Build that wall, build that wall.

Just for the pedants, yes Hadrian's Wall does not mark the boundary between Scotland and England.

[d000hg]

Nicola is talking up Indyref2 this week and seems to be trying to formally launch a new campaign.
We can take as read Scotland would want to join the EU and all other guff aside about, how would a land border with England possibly work?