When you look at the freight ferry routes, it looks like most Irish stuff goes to Northern Ireland and thence to the UK, Belfast/Liverpool.
The ports with direct freight link to the continent seem very small. Even the ports with a link to the UK are small.
Now that, I suppose, is the real reason they want a "frictionless" border. They would have massive expenditure to increase their port sizes.
So, post no deal Brexit, they are f***ed.
They really need to be more polite to us.