Well, that’s more to do with non-EU people (borderless within Europe).
My crystal balls…
Brexit in name only - effectively the largest self-inflicted wound possible. Leave with current deal (DUP will abstain, Tories will back). Then spend the next two years tightening back into the single market, but on worse terms than being a member (rule bound, no say in rules). Effectively Canada with more rules.
As immigration will be a control point, UK will tighten all borders and eligibility (including for historical Irish). Ireland will be pushed into Schengen and tighter EU.
The various movements (including Brexit) will result in larger EU reform and additional emphasis on workers rights, living conditions, external borders etc. The EU will be tighter integrated at the end of 2019 than the start and Ireland will be in it.
EU will maintain it’s shabby stalemate as China deals with its first recession and the US crashes into another (non-contagious this time) financial crash. Trump effectively ignored as a gagging act is passed with Republican support restricting his use of Twitter.