After Brexit more and more goods are being shipped directly between the two.
Hauliers are increasingly shunning the once-speedier route via the UK.
They say new red tape and costly delays are driving the change.
Going straight from one EU state to another now seems preferable.
And all five ferry operators between Ireland and the continent say they're increasing services to meet demand.
Danish line DFDS says its new route from Rosslare to Dunkirk is pretty much full, even though the crossing takes 23 hours.
Glenn Carr is general manager at Rosslare port:
"What we've seen in the first two weeks, at the moment, is a 500 percent increase in freight units through Rosslare port to mainland Europe when compared to the same time last year. So, there's been a phenomenal increase in demand.
It's a similar story at the French port of Cherbourg.
Operators expect it to handle 9,000 trucks in January.
Normally a quiet month, that's around a quarter of what it would usually do in a year.
Port General Manager Yannick Millet says it's easy to spot the difference:
"We see Amazon transporters that we weren't seeing at the port, we see trucks of large retailers that weren't coming through the port. So, we do see that there are changes in the supply chain."
Cherbourg now plans to focus on Irish trade, having previously served it and the UK about equally.
To meet demand, ferry lines are switching some services from the UK, or adding larger vessels.
The question now is whether the change is permanent, or just a passing fad while post-Brexit trade beds down.
Rosslare and Cherbourg will be hoping this is the new normal.