Revenue has confirmed that its new Customs Roll on-Roll off Service is operational, as administrative procedures for freight kick in following the end of the Brexit transition period.

The service enables transport companies to get a Pre-Boarding Notification (PBN) before the goods they are moving start their journey.

A PBN is needed before a lorry can get onto a ferry to cross the Irish Sea, as the UK is now considered a third country for trade purposes.

This morning ferry operator Stena said that six freight loads bound for Ireland have had to be turned away at Holyhead as they did not have the correct references.

In a tweet, the company appealed to freight operators to ensure they had their PBNs ready ahead of check-in for the sailing.

The new Revenue operated Customs RoRo Service also enables transport companies and drivers to check their PBN has "good to proceed to check-in" status, before moving to the departure port.

They can also use it to see if they are able to exit the port or have to call to customs on disembarking in Ireland.

Should they need to call to customs, the Customs RoRo Service can also be used to self check-in.

"Accessed via www.revenue.ie/roro, the Customs RoRo Service is designed to optimise the efficient movement and, where necessary, control of goods and vehicles when moved by ferry between Ireland and Great Britain," Revenue said in a statement.

Revenue has also provided key information and guidance for transport companies and truck drivers to assist them in using the new service, including a guidance video and leaflets in 11 languages.

"Customs formalities and procedures is a clear example of how Brexit has altered trade flows," said Simon McKeever, chief executive of the Irish Exporters' Association.

"We expect that the real impact will be evident on Monday when trade flows are back in full swing following the Christmas and New Year period."

"The recently agreed EU-UK Trade and Cooperation Agreement has alleviated the impact of Brexit to an extent, however we have always made the point that changes are inevitable, irrespective of whether an agreement on trade is reached."