Exports from Britain to Ireland fell by just over 47% in the first quarter of the year - the first period in which the UK had fully left the EU single market and Customs Union.
It was the biggest percentage fall in exports to any of the UK's top export countries recorded by the Office for National Statistics, Britain's official statistics agency.
Imports from Ireland to the UK fell by a much more modest 4.4% during the same period.
In value terms, exports to Ireland fell by £1 billion during the first three months of the year, while the value of imports from Ireland declined by £0.1 billion in the same period.
Chemicals and food were the sectors that saw the steepest declines in exports from Britain to Ireland.
However, the fall in exports in the first quarter is distorted by a corresponding rise in the final quarter of 2020.
Firms stockpiled to avoid supply problems anticipated over the end of the Brexit transition period towards the end of 2020.
Compared with 2019 - the last year in which there was no Covid effect to distort the numbers - trade covering the six months period of the fourth quarter of 2020 and the first quarter of 2021 declined by 3.8%.
In value terms, German imports to the UK fell by the biggest amount - down £1.7 billion (or 30%) in the first quarter.
The biggest component in this was cars and car parts, which fell by £0.7 billion.
The automotive sector was hit by non-Brexit factors. These were notably Covid 19, which led to the closure of car-showrooms in the UK, and by the global semi-conductor shortage, which has led to a slowdown in manufacturing output in car plants across the planet.
However, since the start of the Covid pandemic in the second quarter of 2020, China has overtaken Germany as the UK's biggest source of imports.
In overall terms, trade between the UK and EU was one fifth smaller than it was during the comparable period in the first quarter of 2019.
Exports to the EU from the UK were down £14 billion (21.7%) while imports from the EU were down by £7 billion (18.1%) during the period.