"but if the view is that a hard border would create this,"
A hard border goes far belong nationalism and bigotry, it would severely effect the economies of both sides
Psychologically, the lack of a visible border has done much to bring together both sides - you might believe Ireland to be one country if you allow your attention to slip
Recently the percentage gap between the two communities has almost reached level pegging and the will for a United Ireland has made a United Ireland inevitable in the foreseeable future
That the transition is a peaceful one very much depends on what happens after Brexit
As far as history is concerned - the violence of Northern Ireland has everything to do with the creation of two unequal communities rather than what the Normans did, which is why the recent troubles rose from brutalised Civil Rights marches rather than nationalist or religious fervour
Robert Kee explains it well in his books
Jim