There are great concerns over how cross-border trade will be impacted if the UK "steps into the unknown" and leaves the EU, one harbour boss has said.
Warrenpoint Harbour is undergoing a £2.5m expansion, with Quinn Cement landing the contract, and creating 20 new jobs.
The deal comes after the Belfast Telegraph revealed last year that the Co Down port was seeking firms to construct a huge multi-silo cement site.
And business at the port is on the up, with overall tonnage - the amount of goods handled - up 4% in 2015, rising to 3.2 million.
But Kieran Grant, the Harbour's finance director, says the majority of its customers are concerned over the impact a Brexit could have on business. Around 50% of imports and exports through Warrenpoint Harbour begin or end their journey in the Republic. He said one concern was the prospect of the return of border controls.
"Stepping into the unknown is causing the greatest concern. There are concerns over paperwork, and logistically. With deliveries, a lorry can do two or three across the border each day.
"But with border controls, it could be stuck there for a day. It could create a concern. They (most customers) would be hopeful, because of the unknown, that we would remain in Europe."
Speaking about the latest deal with Quinn Cement, Mr Grant said: "We are delighted to have been working with Quinn since 2012. Their tonnage has grown considerably. This creates a long-term partnership, and hopefully it allows continued growth of exports."
The Harbour employs 65 staff, and hundreds more workers go through its busy yards daily.
Aside from cement, the port deals with a host of raw materials and products. Quinn's 10-year deal will include building cement storage of 7,500 tonnes dedicated to the export of bulk cement. And work is due to begin immediately, with completion expected in September.
Quinn Cement is also in talks over setting up a second import facility in Great Britain.
Liam McCaffrey, chief executive of Quinn Cement, said: "This is a very significant development for the business, which will allow us to provide an even stronger service offering to our UK customers as well as creating additional local job opportunities servicing the growing UK construction sector."
Peter Conway, chief executive of Warrenpoint Harbour Authority, said the new agreement "consolidates an already excellent business relationship" and also "creates a significant economic benefit for the Harbour".