I agree with @peterma that any likelihood here revolves around The Northern Ireland / Eire issues (Protestant v Catholic) and the historical issues among those factions and the semi-common “enemy” (the English) - Much stored anger in there - whether any of it as anything to do with “Brexit”, or whether that is just the excuse for a good old “Barney” I’m less sure !
In some ways it’s a bit like the ongoing “crusades” which have followed us through the centuries and is currently reaching peaks again - with “Christianity” joining with the Judaism, to “rescue” them from the Muslims ! (Read “steal the oil” )
For those of us a little ambivalent about the whole “religion issue” the futility and madness of these situations is simply breathtaking !
[Edit - I’m not saying civil unrest is impossible on the mainland - but that will be a reaction to the perception among the common people that the “Establishment” is determined to “sell us out” and by whatever means to "remain - tied to the apron strings of Brussels ". THAT - If indeed it comes could be really quite serious !