Mrthdrb

UPDATE: I reformulated my question to stress that my focus is on the influence of customs checks, not on the violence in Northern Ireland in general.

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. Is it?

According to my experience, over 95% of private traffic on the customs border between Sweden and Norway passes through without stopping. I assume that a similar arrangement could be used on the Irish border.

Is there evidence that irregular spot-checking of a small minority of private traffic by customs officers would provoke violence?

Notes:

I'm sure that military posts on the border could easily become targets of violence, which is something that happened during The Troubles, but this is very different from hardly noticeable customs posts that let the great majority of private traffic pass through (which is the case on the border between Sweden and Norway).

I know that Northern Ireland is more violent than the European average, as shown by the murder of Lyra McKee, but this does not prove that lightweight customs checks would lead to increased violence.

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-fu will let you find more information on the violence that was happening 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

After Update
The problem is we don't know if they will be hardly noticeable customs posts or full-blown posts from a few decades ago. If there is actually a firm deal in place then all of this was rampant speculation, but most (myself included) have been focusing on the worst-case scenarios since they had seemed more likely for awhile. I'll kick you over to this question for some other references.

The top answer mentions that legally, they can't just have small irregular spot checks for this one particular border since by WTO rules, they would have to use the exact same methodology for all customs. Now they could argue its only used for land borders, but what about all the traffic crossing the channel then? Again speculation, but I'm assuming they will have to default to greater border security than anyone is comfortable with unless a better deal can be reached.

And as for the violence I referenced, yes the first half of the article revolves around the one particular murder. However, it also states

that police are treating [it] as a terrorist incident.

Violence is happening, and probably will continue to happen, for some time. However, any static 'customs' site of any form will by its very nature be a target for radicals. While it may not 'provoke' violence in the traditional sense, it will aggravate those who are already seeking an excuse for violence.

Fizz also has a great followup.

Regarding the updated question:

Customs posts were regularly targeted by the IRA throughout the Troubles as they viewed them as symbolic of the division between Northern Ireland and the Irish Republic. [...]

The Irish Prime Minister brought a copy of an Irish newspaper edition to the [EU-27] summit dinner to emphasise how far Northern Ireland and Ireland had come since the Troubles. The article shared the story of an IRA bombing at the Newry Customs Office in 1972. Nine people were killed and six injured in the attack when a bomb exploded prematurely on August 22. It was the worst attack on a customs post during the Troubles and the single biggest loss of life of customs officials (four). [...]

Another IRA attack on Cloghoge checkpoint near Newry in May 1992 resulted in the death of one soldier and the wounding of 23 others. [...]

PSNI Chief Constable George Hamilton warned earlier this year that if border posts and security installations were created as a result of a hard Brexit that they would be seen as "fair game" for attack by dissident republicans.

“The last thing we would want is any infrastructure around the border because there is something symbolic about it and it becomes a target for violent dissident republicans," the police chief said.

“Our assessment is that they would be a target because it would be representative of the state and in their minds fair game for attack. I would assume that that assessment is shared by senior politicians and officials who are negotiating Brexit."

The Chairman of the Police Federation for Northern Ireland Mark Lindsay said that border posts would make "sitting ducks" out of police.

“If we are saying in the future that police officers could be deployed to customs posts and other fixed points on a hardened border then they would become static targets. They would in effect become sitting ducks for the terrorists,” he said.

As you can see from the above mini-timeline, attack on the initially unhardened customs post(s) probably contributed to their militarization during the Troubles. The (militarized) border posts that you see in iconic imagery of the later Troubles didn't just happen to be there from the beginning. It was part of PIRA's "Long War" strategy to

To make the Six Counties... ungovernable except by colonial military rule.

It seems a fairly widely held view that the attacks on the customs posts led to increased border militarization, e.g.

Dr. Katy Hayward, a reader in sociology at Queen’s University Belfast, explains how the border quickly became a flashpoint for violence:

“If you look at what happened during the escalation of the Troubles in the early 1970s when it was a customs border, customs officials and posts were targeted by paramilitary organizations, especially the IRA. So then customs officials became accompanied by armed police officers and eventually the British army. These areas would be most affected by any change to the status as they suffer from a legacy of underdevelopment due to being cut through by a border. The impact of conflict and division is only really beginning to be addressed now as a result of European integration and the peace process.”

I'm not sure it's the only reason why the border was hardened, because the British army had its own interests in increasing (its own) border controls to make movement of the IRA more difficult (including their cratering operations of minor roads), so there was probably an unintended synergy that resulted in the increased militarization of the border.

Also keep in mind that all these border Troubles happened while the (theoretically wonderful) Common Travel Area existed between the UK and Ireland (since 1923).

Based on agreements that are not legally binding, the internal borders of the Common Travel Area (CTA) are subject to minimal controls, if any, and can normally be crossed by British and Irish citizens with minimal identity documents with certain exceptions.

Except when terrorist attacks (or wars) come into play.

As a bit of an aside, Politico has an interesting article on Newry today, with some striking pictures like RA planted on a pole on the Irish side and a British (remote) security camera high up a pole on the other side of the border. It's one of those less invisible border crossings...

And speaking those cameras... former Real IRA commander John Connolly suggested those cameras would be attacked by "the local population" in case of a no-deal Brexit. And commenting on Connolly's view:

Security analyst and former Defence Forces officer Tom Clonan, who served at the border during The Troubles, said he also agreed with Connolly’s assessment – but that he believed EU border posts would also be attacked.

“There are elements within the border area that do not want normal policing. So anything like that would be very, very provocative in the border areas – masts, aerials, sensors, or cameras would be seen as very provocative. I believe that that type of infrastructure would be attacked and would be destroyed.

“He and I are on opposite ends of the spectrum. I was a member of Oglaigh Na hÉireann. We were there to uphold law and order in the State and he’s a person right at the other end of the spectrum who took matters into his own hands, but I would agree completely with his analysis.

“The only thing is I would go further in saying that you could put the European Union flag on it, you could but the Red Cross on it, you could put the Starship Federation markings on it, it will be taken down and destroyed.

“It would be felt that the information gathered and data gathered by those EU systems would be shared with the British government and particularly with the HM Revenue and Customs service.”

And to circle back to the OP's Norway-Sweden example, funnily enough, a Norwegian customs officer interviewed by Politico said:

“My advice to the U.K. when they leave the EU is: Don’t build the border station too small, you need plenty of space,” he said.

That advice was given simply with respect to the amount of smuggled stuff that was confiscated and had to be stored... (Terrorism not being an issue at the Sweden-Norway border.)

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).

Note that I am simplifying the Northern Irish conflict for the sake of both clarity and brevity.

Is there evidence that irregular spot-checking of a small minority of private traffic by customs officers would provoke violence?

The question is flawed because it assumes the wrong sequence of events. No one is going to get violent just because someone else's vehicle is being checked.

However, hearing about vehicles being checked at the border makes you think about government control, specifically the UK. This reminds you that the UK currently occipies a region that you may consider to be yours. This anti-UK sentiment, especially when experienced by groups of sufficient size (and means/outrage), is likely going to lead to anti-UK action being taken. This could be peaceful action, but it also could be violent. You just don't know.

All you know is what happened last time.

What you're dealing with here is that a part of the population adamantly rejects the notion that Northern Ireland is the property of the UK. This has lead to violence and acts of terrorism in trying to remove the UK from what they consider to be their Irish property.

Regardless of the correctness of the debate on who should own Northern Ireland; the violence became disproportionate and eventually became the main talking point, distracting from the much less in-your-face discussion on administrative ownership.

The decrease of border control has indirectly improved things in the Northern Irish region by decreasing border checks and therefore decreasing UK governmental presence. This isn't the only thing that improved things (of course the Good Friday agreement is the main key), but decreasing border control is obviously the opposite of increasing it (due to Brexit) in terms of border control, which is why I'm focusing on it.

Think of it this way: when standing in Ireland, trying to look into Northern Ireland, the first thing you see/encounter is the border, enforced by border agents who represent the UK government. That becomes a shining beacon of (perceived) oppression, which puts fuel to the flame of considering Northern Ireland to be "occupied" by the UK and requiring liberation.

Because the borders are no longer actively enforced, there is less of a UK presence visible to the Irish near Northern Ireland, which has lead to a decrease in wanting to remove the UK government. For a completely different reason (open borders), the same result (less UK involvement) was already achieved. Enough so to halt the ongoing violence and make the anti-UK sentiment go dormant.

When Brexit happens, and the UK guards its border again, the same anti-UK sentiment, which still remains in the Irish population but is currently dormant, is liable to flare up again. The increased border control will act as a reminder of the past perception of the UK's "occupation" of Northern Ireland and therefore is liable to spark more outrage again, which could lead to an increase in violence.

Nothing of this is a guarantee. But if something saved you in the past, and now you're going to lose that thing that saved you, it's more than normal to consider the possibility that the thing that threatened you in the past is going to threaten you again.

Think of it this way:

You are happily married. However, you discover that your wife has cheated on you with Ryan Reynolds. After a long process of being hurt and reevaluating the relationship, you decide to continue the marriage since she's not cheating on you anymore and has officially stated her intent to no longer cheat on you. Everything is good again.

A few years later, your wife is going to the premiere of a movie starring Ryan Reynolds, has won a meet and greet with him, and tells you how excited she is that she's going to meet him.
Fearing being cheated on again, you discuss marital fidelity with your wife.

Your neighbor has observed this, and he wonders to himself:

Is there evidence that your spouse meeting Ryan Reynolds leads to them cheating on you? According to my experience, my wife was excited to win a meet and greet with him, and she did not cheat on me.

Why would Michau assume that his wife meeting Ryan Reynolds means that she will be unfaithful to him? Where is the proof?

To a degree, your neighbor is right. You cannot know for a fact that your wife is going to cheat on you this time. But what your neighbor did not know is that there is a dormant dispute that is liable to flare up when Ryan Reynolds is discussed.

To you (and your wife), Ryan Reynolds is a painful memory. When he is brought up in a discussion, that inherently also reminds you of your wife's infidelity. You can never just talk about Ryan Reynolds without thinking about your wife cheating on you.

To your neighbor (and his wife), Ryan Reynolds is an actor. When he is brought up in a discussion, that means you're talking about a movie he starred in. When your neighbor is talking about Ryan Reynolds, he's just talking about Ryan Reynolds, nothing else.

Similarly, when raising the issue of a UK-controlled border, this acts as a reminder of the general feeling of Northern Ireland being occupied.

Last time, this lead to violence (just like how last time, your wife cheated on you with RR). It's not guaranteed to lead to violence this time (just like how your wife is not guaranteed to cheat again just because she meets RR again), but it's a valid concern that this could happen.