Guinness brewers Diageo raised fears with the Government that a post-Brexit hard Border would present a "particularly difficult challenge" to them.
Executives told of their concerns about the potential additional costs in their cross-Border operations if there's a return to customs controls at a meeting with Foreign Affairs Minister Charlie Flanagan.
He was told how Diageo brews beer in Dublin and sends it to Belfast to be canned before it's returned across the Border for export through Dublin Port.
"The prospect of Northern Ireland being outside the Customs Union is a particularly difficult challenge for such an integrated operation," a department official wrote in the minutes of the meeting.
"The additional costs of even a 30-minute delay could be hugely significant for a company with 200 truck movements per week," she added.
Records released under a Freedom of Information request show Diageo wanted to use the meeting to discuss its "unique perspective" on Brexit. In an email the firm estimated it supports around 18,900 jobs in Ireland and generates more than €1bn worth of exports.
The meeting was arranged for October 5, 2016 and the minutes note that even then Diageo's "working assumption" was that there would be a hard Brexit.
This was later largely borne out when British Prime Minister Theresa May set out her strategy for Brexit in January.
Mr Flanagan was told Diageo employs around 1,200 people in Ireland, 500 of whom work in the North.
He outlined the Government's Brexit contingency plan to the Diageo representatives.
A key goal for the Government in the upcoming Brexit negotiations is that there is no return to a hard Border.
Last night, Diageo country director Oliver Loomes said a hard Border "would be very unwelcome" as it has facilities on both sides. He said there are approximately 13,000 Border movements a year associated with those facilities.
"The real harm would be to smaller companies and farms, many of whom are part of our supply chain. They would face enormous disruption and very significant additional costs that could threaten jobs and livelihoods," Mr Loomes warned.
"Diageo's focus is to work with both the British and Irish governments on Brexit, who I know are keen to find a solution on the Irish Border issue."
The British government last night suffered a second Brexit defeat in the House of Lords with members demanding a "meaningful" parliamentary vote on the terms of withdrawal from the EU.
Mrs May's government will seek to overturn the defeat when the Bill returns to the Commons.