“Bloody border posts. It’s a waste of money, a waste of time and a waste of space,” said the port company’s chief executive, Eamonn O’Reilly, as he toured the docks recently. New infrastructure is usually a source of pride here, a sign that the gateway to Ireland’s economy is expanding, but O’Reilly winced at the sight of the parking bays, canopies and warehouse earmarked for trucks from Britain. “It’s a complete pain in the ...” – he fished for a diplomatic word – “in the port”.

The port has spent €30m (£25.5m) and set aside 7.8 hectares (19 acres) in preparation for Britain crashing out of the European Union on 29 March, a day that now looks set to be delayed or even cancelled, leaving the border posts idle or redundant. Yet O’Reilly felt compelled to prepare, just in case.

“I’ve given up trying to double-guess what might happen. What happens day by day in Westminster is just extraordinary.” Should Theresa May squeak her deal through parliament in her third attempt this week, the state-owned port’s border posts may end up as 21st-century versions of the Martello towers, a chain of small forts around Dublin Bay built to repel a Napoleonic invasion that never came.

Should no deal materialise, the port’s preparations will face a mammoth test that will shape Ireland’s post-Brexit fate. Almost 90% of Ireland’s road freight traffic passes through the port, an economic chokepoint that has been overshadowed by focus on the backstop and border with Northern Ireland. Food, medicine, clothes, machinery, horses and other goods and livestock come via Holyhead in north Wales, Liverpool and Heysham in Lancashire.

The equivalent of 13km of trucks and containers is offloaded in less than an hour before dawn, one of four “waves” of vessels that dock daily, said O’Reilly. “A line of steel just pouring off the ships.” About 200,000 of the 1.3m containers that enter the port each year originate outside the EU and require customs checks. Should the UK crash out, that 200,000 will rise to 1m – requiring a significant diversion of resources for a port that is growing fast and utilises land, in terms of tonnes per hectare, almost twice as intensively as Rotterdam.

Hundreds of newly recruited or assigned revenue officers, gardai, customs inspectors, vets and other officials are on standby, part of the Irish government’s attempt to “Brexit-proof” the economy. “I’m old enough to remember when customs posts were taken down,” said O’Reilly, a reference to the early 1990s when the EU single market was created. Brexit has renewed his appreciation for that “massive achievement”, a frictionless wonder facilitated by the EU handling trade rules and myriad red tape.

“Air travel, roaming charges, environmental standards – we take it for granted.” O’Reilly, who has hosted British and Irish ministers at the port, is confident that under no deal goods would keep flowing, albeit more slowly. “Dublin port will have the infrastructure ready. I don’t think state services will be fully manned up. It’s unreasonable to expect that Ireland or any country would be completely manned up at the outset.”

Staffing would expand as needed, he said. Asked if he was personally stockpiling anything, he shook his head.  Not all share O’Reilly’s confidence. John McGrane, the director general of the British Irish Chamber of Commerce, speaking on the sidelines of a Brexit conference at the National University of Ireland in Galway, predicted significant delays for ships and trucks and disruption of supply chains.

“It’s a fantasy to think that in a hard Brexit context that what the ports are doing will be adequate to avoid disruption,” he said. McGrane worried especially about the import of raw materials and the fate of mid-sized agri-food companies.

The taoiseach, Leo Varadkar, has warned Ireland that no country can be fully prepared for no deal. “It is uncharted territory. No country has ever left the EU before.” Some Brexiters reportedly hoped the prospect of gridlock and disruption in Dublin and elsewhere would prompt Irish businesses to pressure Varadkar to compromise on the backstop, the insurance policy to avert a hard border with Northern Ireland that has bedevilled the Brexit deal. That has not happened.

There is consensus in Ireland that the government must stay firm on the backstop. Westminster’s febrile mood was all the more reason to have insurance, said O’Reilly. “We love England. But love and trust are two different things.”

By Rory Carroll Ireland correspondent