New car registrations fell by 17% to 3,916 in September when compared with the same month last year, new figures from the Society of the Irish Motor Industry show.
New registrations for the first nine months of the year have also seen a decline, falling by 10.2% to 128,597.
However, used car imports saw a 23.45% jump during the month to 8,665, and between January and September the rise was 37.73%.
Meanwhile, sales of new electric cars for the first nine months of the year are up 56% year-on-year, with 579 units sold.
Volkswagen is the top-selling brand so far this year, with the Hyundai Tucson the best-selling model.
Light Commercial Vehicle (LCV) registrations were down 9% to 1,457 and year to date are down 14%, with 22,488 units sold.
New Heavy Commercial Vehicles (HGV) declined 30% for the month of September (149) compared with the same month last year (213) and are down 15% year to date.
Commenting on the figures, SIMI Director General Alan Nolan said: "New vehicle registrations remain down in all sectors as we enter the final quarter of sales for 2017.
"We continue to stress to Government officials that Budget 2018 must not carry any negatives that would harm our industry and that the motorist should not be burdened further in the upcoming budget.
"The motor industry is currently experiencing the impact of the UK's Brexit decision that has driven-down the value of sterling and has resulted in increased numbers of used imports, of both cars and commercial vehicles.
"This is having a knock-on effect on the sales of both new cars and commercials and on Irish used vehicle values which have been reducing to compete with imports. The Brexit impact is most obvious in the case of used imported HGV registrations which have this year exceeded the number of new registrations by 10%."