I have read a lot about this and the answer to my question seems to be that one is afraid that a hard border between Ireland and Northern Ireland will lead to a restart of "The Troubles" and violence between both factions.
Is that really realistic? The peace treaty is more than 20 years old, a whole generation has grown up without violence and especially Ireland has prospered during this period.
Is there really a good "hotbed", i.e., social and political circumstances, for new outbursts of violence on the island of Ireland today? In comparison, I have a hard time imagine a hard border somewhere around Alsace-Lorraine would cause violence between the French and the Germans.