Car sales of new 182 vehicles were down 2.75% in July compared to the same month last year, figures show.
Almost 27,000 new cars were sold last month, according to the Society of the Irish Motor Industry (SIMI).
New car registrations were also down on 2017 by 4.09%, as just over 114,000 were handled so far this year.
The total Exchequer contribution from car sales for the first half of 2018 was almost €986m, a 1.9% increase on the same period last year.
The average price of a new car was down 1.9% on June 2017, petrol prices were up 9.7%, diesel prices increased by 12.2% and average motor insurance costs declined 11.5%.
Dublin accounted for the majority of new car registrations, followed by Cork and Limerick.
SIMI's Quarterly Motor Industry Review of 2018 highlights that while Ireland’s economic indicators continue to evolve in a positive manner.
It says consumer spending continues to improve at a steady rate, the depreciation in the value of sterling since the end of 2015 has made life more difficult for the Irish motor trade, with new car sales been displaced by used imports from the UK.
Used car imports have increased by 11.7% in the year to date as the motor industry continues to experience significant uncertainty, with monthly declines in new car registrations figures, reflecting the substantial impact of Brexit according to SIMI.
Looking ahead to the remainder of 2018, Jim Power, Economist and author of the SIMI Report said: “The economic fundamentals that underpin new car registrations look set to remain positive but continued strong growth in used imports will continue to undermine new car sales.
Alan Nolan, Director General, SIMI commented: “The 182 sales period has been a good month with new car registrations only slightly down on the same period last year, while registrations in the commercial sector in particular are showing positive signs of growth, with businesses investing in the renewal of their fleets.
"Electric vehicle registrations have seen a marked increased despite currently only holding small percentage of the marketplace and will continue to grow over the coming years," he said.