New car registrations fell by 10.5% to 17,796 in March when compared with the same month last year, new figures from the Society of the Irish Motor Industry (SIMI) show.
SIMI said both the especially poor weather during March as well as two public holidays resulted in fewer trading days.
According to the SIMI figures, for the first three months of the year new car registrations, at 71,842, are 5.5% lower than they were at the same time in 2017.
New Light Commercial Vehicle registrations (LCV) were up 2% (to 3,147) on March 2017 (3,084) and year-to-date are 5.4% higher at 12,537.
New Heavy Commercial Vehicles (HGV) have declined 11.2% over the same annual monthy comparative period and are down 11.2% year to date.
The SIMI figures also show that although there was a slight fall in the number of imported used cars coming into Ireland in March (down 4.6% to 8,154 when compared with March 2017), overall for the first three months of the year imports are 9.5% ahead of the same period last year at 26,116.
Commenting on the figures, SIMI Director General Alan Nolan said: "March has been a challenging trading month for our Industry with snow days and two public holidays leading to shorter trading weeks, but we also had the Hire-drive deliveries at similar levels to last year to bolster registrations.
"Brexit however remains the dominant issue with Used Car Imports up 9.5% for the First Quarter while New Car Registrations are down by 5.5% over the same period."
Continuing the trend highlighted in January, the diesel market share has reduced from 67% last year to 56% in 2018.
SIMI says this will have implications for the country’s CO2 targets, as the Average CO2 emissions from a New Car (113.2 Gms/Km) in Q1 this year has increased by 1.5 Gms/Km (from 111.7 Gms/Km in Q1 2017).
Mr Nolan said this "would produce a calculated increase of around 2,000 Tons in Annual CO2 for the new cars registered so far this year.
"The Increasing volume of imported used cars in Q1 had average CO2 emissions of 121.1 Gms/Km."