84% of freight vehicle movements into Ireland from Great Britain through Dublin and Rosslare ports since 1 January were "green routed", meaning they did not require any checks on arrival, according to Revenue.
A further 12% were "orange routed" for a documentary or similar check.
The remaining 4% were "red routed" for a physical examination or inspection of goods, although this does not always imply non-compliance.
In total, Revenue said there have been 186,500 vehicle movements since the formal departure of the UK from the EU took effect at the start of the year, exactly six months ago today.
"The significant and permanent change in trading arrangements with Great Britain since 1 January last represents the biggest challenge for trade and business in Ireland in almost 30 years, since the creation of the EU Single Market," said Gerry Harrahill, Revenue Commissioner and Director General of Customs at Revenue.
However, the volume of goods imported from the UK has fallen considerably this year compared to last year and pre-pandemic, down 31% in March and 20% in April, according to the most recent figures from the Central Statistics Office.
Over the past six months, hauliers, freight transport firms and other businesses have complained about the additional red tape and new procedures that they have had to grapple with as a result of Brexit.
Nevertheless, businesses have made enormous progress in adapting to new requirements, Mr Harrahill said, with some changing supply chains or routes to eliminate the need for compliance with customs or other regulation.
In mid-January just 70% of freight vehicles arriving into Ireland from the UK were being green routed.
"Customs formalities and other regulatory requirements and checks now apply to goods moving to, from and through Great Britain," Mr Harrahill said.
"These checks form part of Ireland's responsibility as a member of the EU to maintain the integrity of the Single Market and the Customs Union," he said
"In practical terms this means that goods simply cannot seamlessly move from Great Britain to Ireland as they did when the UK was a member of the EU," he added.
Revenue said that intensive engagement with business and trade bodies has led to insights that have allowed it, the Department of Agriculture and the HSE to streamline processes and improve services.
"While it is not and will not be possible to eliminate the operation of formalities and checks, the enhancements have minimised, in so far as possible, some of the challenges first faced in January last," Mr Harrahill said.
Revenue has urged businesses still experiencing difficulties due to Brexit to engage with it and other agencies.