Bosses at Warrenpoint Harbour said they have pressed their desire for a border that supports frictionless trade during a meeting with Taoiseach Leo Varadkar.
The port, which is located in Co Down, close to the border, said over 80% of its seaward trade is to and from Great Britain.
Around 40% of cargo handled last year either started or ended its journey in the Republic.
Chief executive Clare Guinness, who hosted a visit from the Taoiseach yesterday, said it had been "a welcome opportunity to meet with the leader of our closest trading partner".
"We used the meeting to press on Mr Varadkar, our desire for a border that supports frictionless trade, ensuring that it can continue to flow and grow.
"While this is our preferred position, we are actively formulating plans to deal with whatever scenario results from the Brexit negotiations."
Meanwhile, EU Brexit Chief Negotiator Michel Barnier has held meetings with business bodies Manufacturing NI, Retail NI and Hospitality Ulster during his visit to Northern Ireland.
In a joint statement, the organisations said: "We stressed to Mr Barnier that there is a need to find a way which ensures no border in Ireland or between these islands in the interest of our economy and the hundreds of thousands of people across Northern Ireland who work in our businesses."