On a newly constructed quayside in Dublin port last Friday, a bottle of champagne was swung, struck and broken across the bow of the MV Celene. In true maritime fashion, the bottle broke first time — if it doesn’t, superstitious sailors consider it to be a bad omen.

The 235-metre-long roll-on roll-off ferry was purpose-built in South Korea’s Hyundai Mipo Shipyard and it can carry 600 trucks and their trailers at a time. If those trucks were parked end-to-end, they would stretch for about eight kilometres. To accommodate the new behemoth, Dublin Port Authority built three kilometres of berths. Business is booming, and so too are Ireland’s exports.

More than a century ago, on May 31 1911, the RMS Titanic was launched too at an Irish harbour, some 120 kilometres to the north, at the Harland and Wolff shipyard in Belfast. Back then, Ireland was a single entity and part of the United Kingdom. The border between Northern and Ireland and what is now the Republic of Ireland simply didn’t exist.

Less than a year later, the Titanic was at the bottom of the Atlantic and urban legend has it that the bottle of champagne used at its launch didn’t break. For the record, none of the White Star ships in that series were launched stern first, so that myth can be busted.

Already though, the MV Celene has earned the nickname “Brexit Buster”. No sooner was she launched than she was loaded up with trucks and sailed away to Rotterdam. It’s all part of a regular service that will see her ply between Dublin and the ports of Rotterdam and Zeebrugge.

So why “Brexit Buster”?

Let’s put it this way: The launch ceremony was so important for the Belgian, Dutch and Irish business communities that Ireland’s Taoiseach — Prime Minister — Leo Varadkar was present to re-iterate that no matter what happens to that border once Britain leaves the European Union (EU) on March 29 next, it’s business as usual between Ireland and the rest of the EU on the other side of the island of Britain.

Before the launch of the MV Celene and with a couple of limited exceptions, most trucks carrying Ireland’s exports used the “land-bridge route”. That refers to a three-hour ferry journey across the Irish Sea to ports in Wales, then a six-to-eight or drive to cross-Channel ferry ports on the south of England, and then onwards across the continent proper to their final destinations.

With the EU guaranteeing the free movement of goods, services and people across its borders, the land-bridge route works pretty smoothly, rough weather permitting. But Brexit means that’s no more plain sailing, and those Channel ports — on the English side at least — will face customs delays.

And this is where the very notion of a “customs union” comes into play. It’s a phrase that will become critical over these next few weeks, and the very concept itself could potentially see UK Prime Minister Theresa May suffer irreparable damage to her leadership. To keep the maritime analogy going, following a defeat on in the British House of Lords on Brexit legislation on her wanting to leave the customs union, her premiership has a very serious leak.

The customs union is fundamental to Brexit and the EU. All 28 current members apply the same tariffs on imported goods from the rest of the world and, typically, eliminate them entirely for trade. This means costly and time-consuming customs checks are eliminated. Asian shipping containers arriving at Felixstowe or Rotterdam, for example, need only pass through customs once before their contents head to markets all over Europe. Lorries passing between Dover and Calais avoid delay entirely.

These checks also make sure the goods conform with safety standards, and checks are also carried out for security and immigration purposes.

Right now, because Ireland, the UK and the other 26 EU nations are in a customs union, there are no checks. If the UK leaves, there must be checks — it’s as simple as that. A strict customs regime at the UK’s Channel ports or between Northern Ireland and Ireland would lead to delays which will be costly for business and disruptive for travellers. Just-in-time supply chains in industries such as carmaking could suffer. And an Irish peace process built around the principle of entirely unfettered travel between north and south could be jeopardised.

Come Brexit Day, with no customs union in place, there will be traffic jams of 30 kilometres stretching back from the ports, maritime officials say. And that’s on ports on both sides, meaning everything from car parts and cabbage to beans and bearings are delayed. That means lost production, lost sales, lost jobs and shortages in UK supermarket shelves — and higher prices.

May is determined that the UK will not remain in the customs union and she wants the UK to be free to make trade deals with other nations around the world, just like it did when the Titanic was plying the Atlantic — and we all know how that turned out.

But with the clock ticking, the customs union is becoming critical, and she faces a Commons vote at Westminster on the issue next week. Already, 10 Conservative MPs have said they will break ranks and vote with the opposition, meaning another crippling defeat is on the cards — and she’s steaming straight for the iceberg.

The options are limited. May’s Whitehall officials are have outlined two alternatives to preserve the freedom of life outside a customs union while limiting the disruption from leaving.

The first is called “a highly streamlined customs arrangement”, relies on technology and goodwill to limit the impact of new checks. A “trusted trader” scheme and exemptions for small firms in Northern Ireland are intended to put the onus on European business to police itself but would do little to help with other international trade.

The EU says that won’t work.

The second proposal, a “new customs partnership with the EU”, is more ambitious and would see the UK continue to act as if it were in a customs union when dealing with imports from elsewhere. If they are bound for EU markets, the appropriate tariffs would be collected and passed on.

And the EU says that won’t work either. If you’re out of the EU and the customs union, you can’t have it like you were.

What’s more, the Irish, who have a EU veto and have the full backing of the EU Commission — keeping the Irish border open is one of the three fundamental that include the rights of EU citizens in the UK and visa versa, and the Brits’ monetary dies for leaving — don’t believe for one minute that the British plans are workable.

That’s why they’re putting their money where their mouth is, are expanding Dublin port and launching direct roll-on roll-off ferries to Europe with vessels like the good ship ‘Brexit Buster’.