A local councillor in north county Dublin fears traffic chaos if Dublin Port has to activate its Brexit plan.
Truckers face extra customs checks by the end of the month if there's no agreement between the UK and the EU.
That could lead to tailbacks at Dublin Port.
A backup plan involving port management, Gardaí, Dublin City Council traffic management and Transport Infrastructure Ireland was published last month.
"Operation Purge" would see lorries turn around and go back out the Port Tunnel if there was no room for them at the port.
But Labour councillor Duncan Smith says if that happens at rush hour, traffic will be a disaster all in north county Dublin.
"It's going to lead to further chaos," he said. "At the moment, at peak hours, we already have staggered entry into the Port Tunnel -- and this isn't just for trucks or private cars -- there's an awful lot of public transport going through that."
"Express Dublin Bus services use the Port Tunnel. Private coaches from Rush, Lusk, Skerries, Balbriggan and beyond use the Port Tunnel. This is a very very important public transport link for people," he said.
"We're already at the point where you can be waiting outside the Port Tunnel for 15, 20 minutes, half an hour in the morning just to get through it -- and then you've to face traffic on the other side. It's extremely worrying what we could be facing on the 31st of October," he said.
"We've been dealing with Brexit as an abstract now for three years -- we're getting to the point now where this could be a reality."
He says ferry departures may need to be rescheduled to avoid rush-hour.