One inaccuracy is the "illegal movements of goods" and "illegal shipments of duty goods" notion. The movement of duty goods across the border, including for commercial/industrial purposes, is legal. In the duty circumvention situation the illegal act happens at the sale where you aren't following the regulations for sale and duty of fuel/alcohol/etc, not at the movement/shipment across the border.

The second issue is "tax border". WTF is a "tax border" ? That's not a legal term, and that's not even a well-understood notion from 'industry slang'. It's a self-invented bunch of words with unclear definition that makes it unreasonable to discuss whether "tax border between Ireland and Northern Ireland" exists or not. IMHO however you would define "tax border", the RoI-NI has/hasn't a "tax border" if and only if two neighbouring counties in a single USA state that have different sales tax jurisdiction have a "tax border" between them.

I have neither upvoted nor downvoted this. But I'm not sure your "tax border" terminology is widely used. Of course there is a "tax border" like that between almost any two EU countries. Not only is excise likely different but VAT as well.

By “tax border” I did not intend to use an academic term, merely to describe a border at which taxes are collected.

@Peteris You said: “One inaccuracy is the "illegal movements of goods" and "illegal shipments of duty goods" notion. The movement of duty goods across the border, including for commercial/industrial purposes, is legal. ...the illegal act happens at the sale” (your emphasis). This is easily disproven by the interdiction of shipments of fuel across the border in milk tankers. The fuel has not yet been resold. By your logic the shipment is legal. And yet it was interdicted and the perpetrators prosecuted.

@Peteris I think perhaps you are talking about the law surrounding duty-suspension regimes? Even so the nature of illegal cross-border shipments is abundantly clear without descending into the details of duty-suspension regimes.

"a border at which taxes are collected": where is the tax collected?

Physically? Away from the border using electronic system for reasons of efficiency and because the border cannot be hardened because of political tension. Many borders have physical excise collection/detection posts but these are not possible on the island of Ireland. Logically, the tax is collected per the description in my answer.

Although I can see where you are trying to get, I think your labeling is somewhat contentious. Excise border, or tax border could easily be mistaken by some kind of physical border, particularly given that you asked specifically for Ireland-NI. Continued and organized fraud is likely more of a cross-border police issue than any physicality of a "border". This type of problem could even apply to one country where different regions (states, autonomous, etc.) have different duties or VAT. Calling it a border is kind of a stretch, particularly given that that is a very used word nowadays.

There is a border between two states. Tax becomes due at it: in this case excise duty and VAT. Given that tax is due at the border I used the descriptor “tax border.” I am open to using another descriptor, but I suspect the meaning would be unchanged. What descriptor would you use?

Excise tax doesn't become due at the border. It's a consumption tax and is not due until it is sold, exactly like VAT. If the product is lost/destroyed before being sold Excise tax never becomes payable.

From the UK Government’s website (link at end): “The duty falls due at the time when the goods leave any duty suspension arrangements” I am happy to change my wording from “tax becomes due” to “duty falls due”. Under a distance selling arrangement (for example) duty falls due when a delivery across the border is made. gov.uk/government/publications/…

@Ben Again, I understand what you are describing. I just think calling a border is adding needless confusion to the matter. I believe the expression duty (or perhaps tariff in a trade agreement) already assumes some kind of change in jurisdiction. And this is likely the most common expression everywhere: excise duties. The question could be reformulated to: How does NI and Ireland manage (or regulate) the different excise duties in cross border trade? Not only you would be using a more common excise expression but the word "border" would be used in a more familiar context.

No. The expression “duty” does not imply a change of jurisdiction. You pay duty when you fill up your car at the petrol station down the street. At borders you have a specific set of circumstances where duty falls due because of the transit across the border. Hence my question.

No, but it implies that is unique to a jurisdiction. That is, the jurisdiction (whatever it might be) choses the duty. That's why I formulated the question in three points: a) excise duty, b) in cross border trade, c) in the specific case of Ireland-NI. But I won't insist on it. I just think you could have avoided a flood of misinterpretation with different wording.

You said: “I believe the expression duty already assumes... some kind of change of jurisdiction.” That a law (for example the law setting the duty rate) exists within a jurisdiction is a truism & need not be stated. It probably sounds like I’m being a pedant, but I’m not. You have presented me with a statement & I am refuting it. In Ireland you have two excise jurisdictions bordering one another. There is no legal agreement in place between Ireland & the UK to eliminate the excise border, so an excise boundary or border exists. Excise is a tax. Hence my descriptor “tax border.”