The Taoiseach says he is concerned about traffic jams at English ports holding up lorries bound for Ireland post-Brexit.
Micheál Martin visited Dublin Port on Monday morning to inspect new facilities which will be put to use when the transition period for the UK to leave the EU ends on January 1.
He said it was a "fundamental change that people need to get into their heads."
"That which ordinarily went through here seamlessly would now have to go through a process.
"The aim of all of our preparation is to make it as fast as we possibly can.
"One concern I've had is that maybe there is a complacency among some SMEs out there that everything will be okay.
"It will be different, it will not be as seamless as it once was."
He says that the scaling up of similar facilities in ports like Holyhead is cause for concern due to the sheer size needed and lorry queues are looking likely after January.
"I am concerned, and I know the Department of Transport has been working with the commercial sector in terms of perhaps looking at alternative direct routes," he said.
"Stenaline and others have increased their capacity in terms of direct routes and businesses have been encouraged to trial that, to engage with your market in your sector and pilot that as an alternative, because we can get some sense of the logistics here.
"In the UK, they are bigger again and that is a concern, it is a worry.
"We were making really big preparations here, in the UK they have been preparing and have been allocating resources to their side, but it will be challenging and of course we're concerned.
"Economically it's the biggest change in 50 years."
The Taoiseach says he is "more confident" about dispute mechanisms.
"I think there's a lot more in common between the United Kingdom and Europe than that which divides us, in a trading context.
"So I don't see any huge immediate divergence. There may be some issues, that the trade deal deals with.
"Actually I think once we get a deal agreed, I think that would allow for a degree of settling down, building trust because we have to, for all to the people we represent, do that."
The Taoiseach noted that advanced preparation is key for Irish companies or businesses to speed up the process when trading with the UK post-Brexit.
"There's been time for people to get ready," he said.
"The deadline is the first of January and the bottom line is the UK will be a third country, in the context of the European Union. So, that is the reality.
"We've had a transition year period. So those people just need to double down."
He confirmed he is still optimistic of a trade deal between the UK and EU and that most issues have been agreed on, but says all negotiations will come to a "crunch time".