The chance is very low given the amount of unionists, but that won't stop this being used for what its worth to force re-assessments of agreements etc. Expect the Northern Ireland government to be given a slew of new devolved powers etc. over the next few years as concessions to prevent growing calls for it.

What are the odds of violence popping up again?

We've recently had the threat level for that type of violence raised.

Depends on the number of unionists who are more pro-EU than pro-UK. It's not as likely as Scotland leaving (which is very likely), but I suppose it could happen.