Will Brexit reignite tensions in Northern Ireland?

The United Kingdom falling apart is a very clear danger, at least that is what was in the air when we visited Belfast before the UK elections in December.  Unionists don't want to have a border on the Irish Sea, Irish nationalists want free access to the Republic of Ireland, and there is no solution that appeases both parties.  The question of the border unearthed tensions kept under control by the Good Friday Agreement, and the centuries-old conflict is a major issue to be resolved until the 31 December deadline when the transitional period is said to end, otherwise, violence could erupt once more. 

What do the locals expect to happen in a post-Brexit Northern Ireland?