New car registrations fell by 10% in the first half of the year compared to 2016, according to Motorcheck.ie.
Nearly 91,000 cars were registered by June compared to just over 101,200 in the first six months of 2016.
The number of commercial vehicles registered fell by 14% to just under 19,000 units.
Industry experts say an increase in people importing used cars from the UK after Brexit has contributed to the decline.
According to their figures, imports were up 41% for the first half of the year.