The political situation in Northern Ireland has reached a low point this decade with the Stormont deadlock:
[Northern Ireland] has been without an executive since January 2017, when the governing parties - the DUP and Sinn Féin - split in a bitter row.
Northern Ireland's biggest political crisis for more than a decade has left Stormont in limbo. [...]
Take the big sticking point of Sinn Féin's demand for legislation to give official status to the Irish language in Northern Ireland.
It's true that other countries with deep divisions like Belgium have had similar deadlocks, one lasting 541 days, yet no violence broke out. But clearly the situation in Northern Ireland is not just a case of "up-and-up", and I would argue that the breakdown of the power-sharing scheme is a bad omen because it may entail return of more direct rule from Westminster, which was one of the rallying points of nationalists (to fight against). In the words of Sinn Fein vice-president, Michelle O'Neil (19 March 2019)
"The British Government and Michael Gove, a long-term opponent of the Good Friday Agreement, are playing to the unionist demand for unrestrained British direct rule.
"I cannot overstate the grave implications if the British Government follows through on these threats."
I can't think of party with [former] ties to paramilitary forces threatening "grave implications" not being taken as likelihood of a return to some kind of violence.
Sinn Féin still does not send MPs to Westminster because they don't want to swear the vote of allegiance to the Queen. (This is quite unlike what happens in Belgium, i.e. the political rift in Northern Ireland is deeper; at least in Belgium they're no longer arguing about the official status of their languages, as far as I can tell.)
Furthermore, the last border post between the Republic and Northern Ireland was dismantled only in 2006; that's not long ago.