If the NI remains as is & Brexit proceeds, then the net effect for Ireland will be defacto re-unification, IMO.
The NI middle class & small businesses benefit greatly from the status quo and there are significant economic synergies
in retaining current services sharing such as electricity, water, Ag services etc.
Young people in the South, however, are not that sold on the notion of unification because as children they grew up in fear of violence boiling over into the South.(a more serious and ongoing version of today's anti-terror "Lock Up" in our local State School) My niece is currently doing her PHD on particular areas of youth cross border relations and the residual effects of past community violence and personal trauma.
IMO, if the customs border post Brexit is in the Irish sea, then it will only be a matter of time until the Scots exit too.
That said , the UK authorities will be very aware of this and I would not be surprised if there was a U-turn on Brexit
once the costs & national interest consequences are worked through.
Ireland is currently diversifying trade to position itself for a hard Brexit but there are many Brexit sceptics in Ireland
who think the Poms will capitulate before handing over the lollies and instead cut a deal with Brussels to limit immigration which was the main motive for Brexit anyway. There seems to be growing support in Germany & France to limit labour mobility while retaining full free trade.
There has been some idle chatter around the Border areas that Northern Ireland should seperate as a country which would kill two birds with the one stone:
-Avoid Unification with the Catholics in the South
-remain in the EU & join the EZ with a soft North/South border and yet be a member of the Commonweallth.
This would alleviate Protestant fears of the South, retain the Queen to appease the royalists and compete favourably with the South as a Tax Haven and continue to capitalise on its growing tourist trade.
