Ireland and the UK have both signed into Schengen cooperation agreements. They enjoy the status of partial participants.

This participation requires the free movement of people, therefore, if Brexit is to be taken literally, the UK will also exit Schengen cooperation. Under that scenario, the new Northern Ireland - Republic of Ireland border will enter a statute similar to the Norwegian - Russian border. By default this means thorough border controls.

Some have suggested a special agreement between the UK and Ireland to leave the border as it is. However, this puts Ireland's partial participation into Schengen in question.

I refrain from issuing any definitive prediction on this matter, because it really is a thorny issue.

You might find me At The Edge Of Time.