New figures from the Central Statistics Office show that the number of new private cars licensed for the first time in August fell by 5.2% compared to the same time last year.
The CSO said a total of 7,681 new private cars were licensed for the first time last month, down from the figure of 8,105 in August 2017.
Meanwhile a total of 8,586 of used or imported private cars were licensed last month, an increase of 4.5% on the same month last year.
The August figures continue the trend that began shortly after Britain voted to leave the European Union, as the subsequent slump in the value of sterling made it more attractive to buy and import vehicles from the UK.
Today's figures also show a 9.5% increase in the number of new goods vehicles licensed in August of this year, bringing the total to 2,277.
In the first eight months of 2018, 111,461 new private cars were licensed, down 4.5% compared with the same time last year, while the number of used or imported private cars licensed increased by 10.4%.
Today's figures also reveal that Volkswagen was the most popular make of new private cars licensed in August, followed by Ford, Nissan, Toyota and Skoda.
Together these five makes represent 43.4% of all new private cars licensed in August.
The CSO said that during the first eight months of 2018, 54% of new private cars licensed were diesel powered.