Figures show 6,121 new cars were sold in April, a decline of 9% on April last year.
So far this year, 45,581 new cars have been sold, 13% down on this time last year (52,600).
In the commercial vehicle sector, which is often a barometer of business activity in Ireland, sales of vans and light trucks were up 10% in April, compared to April last year, but they are down 9% for the year.
Truck sales were down 22% compared to April last year and up 2% for the year.
Despite this, the average new car now costs €24,483, €2,621 less than 2007.
Alan Nolan, Society of the Irish Motor Industry (SIMI) Director General, said: "Given that 130,000 new cars are sold in a 'normal' year, the market is operating at about half of normal levels. This is unsustainable.
"With consumer confidence and spending still showing no real signs of recovering anytime soon, and with much uncertainty among the public sector around Croke Park 2, this has impacted on the new car market.
"The good news for people thinking of buying a car is that new car prices have fallen. The average price of a new car today is 10% cheaper than in 2007 and consumers are encouraged to shop around to avail of the many deals that are on offer."