The full implications of Brexit on established food supply chains between Ireland and the UK, and the UK and the European continent, have been painfully revealed in recent days.

How food moves from Britain to Northern Ireland, Ireland to Britain, and Ireland to Europe are now critical elements in the final throes of the negotiations.

This week the Danish shipping line DFDS announced a new daily service between Rosslare Europort and Dunkirk that will bypass Britain and give Irish food exporters to Europe another seaborne alternative to the land bridge.

So, as we get closer to 31 December, a lot of the reality of Brexit is now moving from the abstract to the kitchen.

On Tuesday, RTÉ News broke the story of a potential two-way ban between Britain and the island of Ireland on sausages, mince and other prepared non-frozen animal-based meals.

Minister for Agriculture Charlie McConalogue said this was a "reflection of the many issues which Brexit is causing".

However, there is a lot of suspended animation here. There are some foreboding elements linked to both the Northern Ireland Protocol and the free trade negotiations that may reflect hardball negotiation tactics.

In other words, there are possible outcomes that might make the whole picture a lot worse, or they could still be potentially resolved by behind-the-scenes discussions that might mitigate some real potential hazards.

Take the sausage revelation.

The EU restricts and prohibits a range of non-frozen meat products, or products of animal origin, from third countries because of traceability and consumer health concerns.

That will mean, on paper, that such products made in the UK and delivered to Northern Ireland supermarkets will be banned from 1 January, since the North will be maintaining EU food safety rules while the rest of the UK will not.

Some products are banned outright; for others there is simply no animal health certificate to accompany them. Minced poultry, either frozen or chilled, does not have such a certificate. For minced beef and pork, the certificate requires that they be frozen and not chilled.

Chilled sausages also need to be frozen and meat that is cut with added products from another country (even if in the EU) are prohibited because only the country in which the animals were slaughtered can provide the health guarantees to certify the product.

What made the issue more acute is that the UK signaled they would copy the EU's food safety regime, meaning they too would impose a ban on such products coming from Ireland (and the rest of the EU).

That threw into disarray the lucrative trade in prepared, chilled meals from Ireland to GB.

"Up until recently, the UK had indicated that it wasn't its intention to apply the same restrictions on food coming into the UK as the EU apply in terms of imports into the EU," Mr McConalogue told RTÉ's Morning Ireland.

"However, in recent weeks, the UK indicated that it will reciprocate the same requirements that the EU has for certifying imports. That obviously now raises a question mark over chilled and non-frozen processed meat exports to the UK."

Irish officials were aware that the UK would introduce controls, but not immediately. They seemed taken aback by the notion that the UK would simply copy EU rules and then impose them on EU imports.

"The signals from London were that the Brits weren't going to apply controls for the first few months," says one source familiar with the issue. "So, maybe people put too much comfort on that. Certainly, the impression was created that there wouldn't be severe restrictions. Now it's becoming clearer this will be a two-way thing."

Have the UK definitely decided they will mirror the EU's food safety rule book?

On 18 November, Emily Miles, CEO of the UK's Food Standards Agency (FSA), told the agency's board that under a new system to monitor food imports from Ireland and other EU countries, any "high-risk food and feed" would already require pre-notification from 7 December, while they would have to be accompanied by the "relevant certification" from 1 April 2021.

What would be the point of Brexit if the UK were simply to copy the EU's rulebook?

"From 1 July 2021, all imported high-risk food and feed from the EU [including Ireland] will be subject to increased risk-based controls and physical checks, though the frequency of such checks has still to be determined," she said.

However, there are indications that the UK has not yet formally and finally proposed to copy the EU's own restrictive food safety regime and simply turn it against EU products entering the UK.

Firstly, the idea itself is somewhat sensitive, if not contradictory. What would be the point of Brexit if the UK were simply to copy the EU's rulebook?

UK sources stress that, were the UK to follow that course, there would be a difference between London taking a sovereign decision to do as the EU does, compared to London aligning its laws with the EU's laws.

The source stresses that, despite what the FSA believes, no final decision has yet been taken to mirror EU rules.

That may be because it is all linked to the food issues at the heart of the current free trade negotiations.

These include what kind of food safety and animal health regime the UK introduces from 1 January, and in turn the issue of being granted a so-called third country listing from the European Commission, which will designate the UK a safe country from which to import food.

This is the issue which blew up into the claim that Brussels was going to impose a food blockade on Northern Ireland.

Put simply, many of these issues may yet get resolved in the free trade agreement. The FTA may well include a legal framework for the EU and UK to recognise each other's food standards.

They are also the subject of intensive discussions within the Joint Committee, which is currently trying to resolve such conundrums within the Northern Ireland Protocol. 

The risk of a two-way sausage ban is down to the EU signaling that sausages and so on coming from a third country (Britain) cannot enter the EU's single market (Northern Ireland), followed by a reciprocal threat by the UK to do the same for sausages from the Republic.

While all such food complications are being dealt with in the Joint Committee, a reciprocal UK threat does no harm when it comes to leverage. UK sources are adamant that no such retaliatory threat was there or intended.

But the prospect of the UK mirroring the EU's food safety rules is causing real concern elsewhere, not least on the land bridge.

Every year 150,000 truck crossings carry three million tonnes of freight from Ireland to the rest of the single market across the UK land bridge (and back again).

While the UK was an EU member state this was a seamless process and facilitated just-in-time agri-food trade flows. The journey time from Dublin to Calais was an average of 13 hours.

Ireland quickly recognised that when the UK left the EU that model would be upended. There might be checks and controls at Holyhead, Irish trucks would get snagged into long queues at Dover, and might even face further delays and checks in Calais.

During the Withdrawal Agreement negotiations, a small team of officials from the departments of Foreign Affairs, trade, agriculture, transport and the Revenue Commissioners held meetings with their Belgian, French and Dutch counterparts and the European Commission to see what might be done to limit the damage.

In February, when all member states agreed Michel Barnier's mandate to start the free trade talks with the UK, a commitment was made that "while preserving the integrity of the Single Market, the envisaged [EU UK] partnership should ensure that issues arising from Ireland's unique geographic situation are addressed".

Dublin wanted to know if the flow of Irish goods through the UK to the continent could somehow remain frictionless. Could there be priority lanes for Irish vehicles heading to Dover? Could any formalities (and costs) related to bringing food back into the EU in Calais be waived?

The problem was unexpectedly made worse by the entry into force of a new, comprehensive set of rules governing food safety and animal health in December last year.

The Official Controls Regulation (OCR) harmonised and strengthened existing EU rules on how live animals, products of animal origin, plants, pesticide residues, animal feed should enter the single market.

In general, post-Brexit, Irish goods rolling across the land bridge would be handled under transit rules: these would be EU goods going to other EU member states, but just crossing through what is about to become a third country.

Despite sympathetic noises from London, there have been no formal arrangements made with the UK that would facilitate Irish trucks going from Holyhead to Dover.

Transit means the goods being placed in sealed trucks and bonded - ie, underwritten by a financial guarantee that they won't go missing during transit - and then registered via the global system for transiting goods known as the NCTS (New Computerised Transit System).

This system allows customs officials at either end to know what is inside the vehicles which are transiting.

However, under the OCR, the rules were tightened so that Irish exporters would still have to pre-notify the veterinary authorities in France, Belgium or the Netherlands if they had any live animals or products of animal origin, even though they had originated in another EU member state.

This would have meant an extra layer of cost, delays and bureaucracy for Irish food being shipped to the continent. The goods would have to be registered with the EU's monitoring system known as TRACES.

Under TRACES, mixed consignments of goods would also have required a different certificate for each type of animal product (a dairy cert for butter, a meat cert meat and so on).

Following detailed talks with the Commission and other member states in June, Brussels agreed to a derogation for Ireland.

Any Irish food products arriving at European ports via the UK could be treated solely under the straightforward NCTS system which would use the data provided to customs authorities to cover a multitude of formalities.

By removing the TRACES obligation, multiple animal health certificates for multiple food products would not be needed, and Irish trucks could be "green-routed" off the ferry and out of the port.

However, the challenges on the UK land bridge itself have remained.

Despite sympathetic noises from London, there have been no formal arrangements made with the UK that would facilitate Irish trucks going from Holyhead to Dover. Priority lanes for Irish trucks have been, understandably, regarded as not entirely feasible, if those trucks are sailing past miles of gridlocked lorries with UK plates.

Even if Irish goods are registered as transiting to the rest of the EU, officials say the process of getting across the land bridge will be "very challenging" and will require between 13 and 16 procedures.

These will include hauliers registering for no fewer than four phone apps: one Irish one, two UK apps (the Goods Vehicle Movement Service, or GVMS, and the Kent Access Permit), and the French Brexit app.

There are concerns that the GVMS app will not be ready on time and Irish officials are unclear if all UK ports will be using them. If the latter two scenarios prevail on 1 January, that means Irish hauliers will need to do some transit paperwork on arrival in Holyhead.

The Kent Access Permit is simply a questionnaire for the driver, meaning it also may not guarantee smooth sailing.

Furthermore, if the UK does decide to mirror EU food safety and animal health rules - as discussed above - that could also have a knock-on effect for transit.

Under EU rules, any third country products of animal origin (or live animals) transiting EU territory must be accompanied by export health certificates.

If that is applied by UK officials, then, by extension, any similar Irish products would - on paper - require the same certificates, even though they are simply crossing the land bridge to EU customers.

If the UK follows through on this, the checks would not take effect until 1 April, and sources have speculated that a pragmatic approach would be adopted. But that would be of scant comfort to Irish hauliers, and the Irish meat industry.

While the UK land bridge has always been quickest, officials are now adamant that the combination of huge administrative burdens and queues on the way to Dover mean the direct ferry route will become a no-brainer.

All of this adds up to a serious rethink of the practicality of the land bridge, until now by far the quickest way of getting goods to the European continent.

New ferry routes have been opening up. Stena Line and Irish Ferries ships will soon go out within an hour of each other on the same day from Dublin and Rosslare Europort to Cherbourg, and the numbers of sailings are due to increase.

As announced on Friday, the new DFDS ferry will offer 13 direct weekly sailings from Rosslare Europort to Dunkirk on the northern French coast each way during peak times.

While the UK land bridge has always been quickest, officials are now adamant that the combination of huge administrative burdens and queues on the way to Dover mean the direct ferry route will become a "no-brainer".

Dublin to Cherbourg is 20 hours, Rosslare to Cherbourg takes between 17 and 18 hours, Rosslare to Le Harve takes 20 to 21 hours, and the new Dunkirk route would take between 24 to 26 hours.

For the UK, whatever happens, as a net importer of food it is almost certain to face disruption of food supply chains.

"Until now you haven't had warehouses full of stock," says Maree Gallagher, a specialist in food law with Covington & Burling, "you have just in time delivery. So, if you run low on cheese or pizza or cauliflower an order goes in and it's delivered with everything else.

"Because the UK has been part of Europe that has just worked, whether it's going to France, or Germany, or Italy or Ireland or Northern Ireland."

For Irish food importers from the continent, the issue is not just one of food safety when it comes to using the land bridge.

Take an Irish retailer which sells a popular chocolate brand.

The chocolate is manufactured in the Netherlands by an international company which operates in the UK to supply the British and Irish markets.

Will Brexit disrupt this supply chain? The milk used in the chocolate is EU milk, so why should there be a problem with this chocolate being sold in Dublin?

In fact, there are several problems, and there is just over a month for importers to figure them out.

If the UK imposes the same traceability rules for food coming from a third country it will have to be notified to the UK food safety authorities.

Then there's the labelling problem. Any goods of animal origin (including milk chocolate) entering the EU will require specific labelling as part of the traceability rules. That label must carry an EU address. However, from 1 January, because the manufacturer is based in the UK, that address will no longer be valid.

What options does the importer of the chocolate have? The product could be imported directly from the Netherlands into Ireland by sea, avoiding the land bridge.

Or the company creates a new address in Ireland and a new label. But changing labels is costly and time consuming and might not be doable between now and 1 January.

All of these issues are painfully poised between the imperatives of the wider Brexit negotiations, the tensions at the heart of the Northern Ireland Protocol, and the challenges of the UK starting a new food regime from scratch.

The UK may decide to mirror EU rules from 1 January, but that does not seem like a feasible long-term plan if it is to conclude its own trade deals with agri-food chapters.

If it therefore diverges from EU rules, then the bonds of food exchange and supply built up over decades between Ireland, Britain and Europe could unravel, and the food component of the Northern Ireland Protocol could flare up once again.

"Anyone who thinks this is just going to be sorted out over the next 30 days is mistaken," says Marie Gallagher of Covington & Burling.

"I think the EU hands are really tied on this. They're not going to be able to change the rules around food safety, traceability, certification over a period of a few weeks just to suit a departing member state."