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My impression is that the EU and most British supporters of a soft Brexit agree that any solution that involves customs border between Ireland and Northern Ireland is dangerous. What is the evidence that a customs border in Northern Ireland, similar to the borders the EU has with Switzerland and Norway would lead to violence?

I have crossed the Norwegian land border with Sweden a couple dozens of times, and in all cases but one I could pass though it without stopping. And in the only case when I was stopped, the officer just wanted to see my ID and didn't even inspect my luggage. I don't understand how a border that is almost invisible for private persons could be a source of a violent conflict.

Of course, I do realise that it looks differently from the perspective of a company that trades across the border, and I imagine that cargo would undergo more thorough checks. But how is it going to incite violence? Is there a reason to believe that company owners could become terrorists just because they need to pay customs duties?

New here, but there's a misconception in your question. The worry over the border isn't about some trade disputes and companies turning ugly over tariffs; this is about the establishment of any border at all between 2 regions of Ireland.

I definitely can't go into all the details but here's a reference to the Wikipedia on The Troubles which is central to this conversation. From there, some simple Google foo will let you find more information on the violence that was happening within a generation ago over this very border. The Good Friday Agreement is a good place to start as well, although there is debate over whether that prevents trade checkpoints or ALL checkpoints. See other questions on this very site.

And as for evidence of violence well.... This happened just a few days ago

To expand on Red Mage's answer, Wikipedia has a timeline of the Northern Ireland Troubles and peace process. To better understand it, it's good to note that the Good Friday Agreement was effective from December 1999.

Since the Wikipedia list is chronological, a good way to visualise the violence is to use the search option in your browser (which adds little yellow stripes in the scrollbar where the search term occurs).

For example, when we search for the word 'shot', we see many more instances before the turn of the millennium than after it. Other frequent terms include 'bomb', 'explosive', 'killed' and 'wound'. Notice that each of these terms are unlikely to be associated with other EU borders, it's not something you'd expect on the peaceful continent of Europe.

Another good way to read into the situation is to follow up on Wikipedia's references. The link to more extensive news articles which add a lot of context. It should also be noted that the list seems properly maintained. For example, the shooting of the journalist around the 2019 Easter weekend is not yet listed on that page because it's not yet confirmed to be related to the Troubles (though as explained in Red Mage's link, the police says the 'New IRA' is likely to be behind it).