The volume of traffic through Dublin Port in the first nine months of this year is down 3.3% compared to the same period last year.
Container traffic is broadly unchanged, down just 0.5%.
However, the impact of Brexit border controls since January of this year is clear with container and trailer traffic to and from Great Britain down 21.2%.
By contrast, traffic to and from Continental ports increased by 36.3%.
Containers and trailers to and from Britain now account for just over half of Dublin Port's traffic compared to 64% pre-Brexit.
Also, the proportion of containers accompanied by a driver has fallen from 32% to 26%.
"This is bad news from a port capacity perspective," Eamon O'Reilly, Dublin Port's chief executive said in a statement.
Another identified trend is that containers are less full than they were pre-Brexit.
According to its statement the reason for this "...is that the average size of a load in a container or trailer has reduced because operational efficiencies which the Single European Market had facilitated in trade with Britain have been removed because of Brexit."
Mr O'Reilly said the only positive thing since Brexit is that much feared delays to traffic had not materialised.
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However, he also said the "dislocation" of traffic to ports in Northern Ireland would be a "permanent feature" and a reversal of what happened when the Single European Market came into being 30 years ago.
Speaking on Morning Ireland, Mr O'Reilly said the same goods are coming in, but they are arriving by different routes.
There has been a remarkable shift in the make up of the business, he added.
Amount of goods handled by Irish ports up 17% in Q2 - CSO
Meanwhile, new figures from the Central Statistics Office show that Irish ports handled nearly 13 million tonnes of goods in the second quarter of this year.
This marked an increase of 17.4% on the same time last year and an increase of 5.1% on the second quarter of 2019.
The CSO said that goods forwarded from these ports amounted to over 4 million tonnes, while a total of 8.6 million tonnes of goods were received.
Today's figures also show that the total number of vessels arriving during the three months from April to June in the seven main Irish ports - Bantry, Cork, Drogheda, Dublin, Rosslare, Shannon and Waterford - increased by 347 (13%).
The gross tonnage of all arriving vessels increased by 7.7%, the CSO added.
Dublin port accounted for 60.3% of all vessel arrivals in Irish ports and for 48.7% of the total tonnage of goods handled in the second quarter.
The CSO noted that Great Britain & Northern Ireland accounted for 32.1% of the total tonnage of goods handled in the main ports by region of trade in Q2 2021, compared with 37.3% during the same time last year.
Other EU countries accounted for 41.9% of the total tonnage of goods handled in the main ports, a 4.1 percentage point increase compared with last year.