Expect longer journey times as transport links at risk post-Brexit
Bus and rail passengers face lengthy delays post-Brexit if customs checks and passport controls are re-introduced on the Border.
The National Transport Authority (NTA) has also warned that publicly funded bus services could be at risk as the Republic and UK grapple with a new way to licence cross-Border routes.
Currently, under EU rules, the NTA can refuse to licence a commercial bus operator if their services impact on a route subsidised by the taxpayer. This is designed to protect the State's investment in services.
But once the UK leaves the EU, these rules no longer apply.
Instead, a new set of procedures will be required, setting out the rationale for granting or refusing an application and rules around how operators could be suspended from the market.
A so-called interbus agreement, which covers bus services from non-EU countries entering EU territories, will require negotiation between governments.
"Currently there's no criteria," a source said. "You could grant everything, or refuse everything."
The concern is that unless there is clarity, the State could be sued by operators who were refused a licence.
Other Brexit issues of concern include the impact Border controls would have on hauliers and motorists.
It would be "impossible" to deliver reliable journey times between Belfast and Dublin, which might also impact on holidaymakers flying from either city.
The UK may also change rules around professional driver hours, giving British firms a competitive advantage, and Border checks might also impact on professional hauliers.
"We have rigorous rules around driving hours," Irish Road Haulage Association president Verona Murphy said. "We're expected to take 45-minute breaks at certain intervals. But if you're getting delayed, the infrastructure isn't there to allow drivers to pull in and rest.
"Delays make a difference between the driver getting home to their bed or sleeping in the truck."
Bus Éireann operates cross-Border services, via Expressway, and it says the Dublin-Belfast route is among the network's busiest. It said it had no indication of any licensing issues.