There was a 5.1% drop to 21,316 in the number of new cars registered during July, when compared with the same month last year.
New figures from the Central Statistics Office also show the number of used imported cars has risen by more than a third (34.8%) to 7,598 over the same comparative timeframe.
The drop-off in new car sales has been anticipated by the industry, following record sales in 2016 that are unlikely to be matched this year.
For the year to July, new car registrations are down by 9.4% to 108,662 when compared with the same time a year ago.
Since January used car imports into Ireland are 44.3% higher than the figure for the first seven months of 2016.
The jump in imports over the past year is largely down to the fall in the value of sterling following the UK's vote to leave the European Union.
The pound has lost around 15% against the euro since last June, making it much more lucrative to import cars from the UK into Ireland.
Today's CSO data on car registrations also show Hyundai was the top selling make in July, while almost two thirds of newly registered cars were diesel.
