The latest new car registration figures from the Society of the Irish Motor Industry show a 5.5% slowdown to 71,805 in the first three months of the year.
This compares to the figure of 75,982 the same time last year.
SIMI's quarterly figures show that the number of imported used cars bought rose by 9.5% to 26,116 compared to the first quarter of 2017.
Meanwhile, new light commercial vehicle registrations were up 5.3% to 12,529, while new heavy commercial vehicles have declined 11.2% to 916.
SIMI's latest Quarterly Motor Industry Review also predicts that new car registrations are set to reach around 120,000 this year - a decline of 8.6% on 2017.
Used car imports by contrast are projected to grow by around 15% to reach 107,470.
Similar to last year Brexit related uncertainty looks set to continue, largely due to the uncertain performance of sterling and the impact of used imports from the UK, SIMI noted.
Today's report also highlights a number of price decreases in the cost of motoring.
It noted that the average price of a new car in the first quarter of 2018 was 1.9% lower than a year earlier.
However consumers are spending more on higher specification cars with the average OMSP (Open Market Sales Price) up 4.1% on the same time last year.
The cost of motor insurance in March of this year was 13.8% lower than it was a year earlier, but average motor insurance costs in March were 37% higher than in March 2013.
SIMI also noted that the cost of fuel decreased slightly in the first three months of the year, with petrol prices down 0.7% and diesel prices down 0.5% last year.
Alan Nolan, Director General of SIMI, said the fall in Irish used car values due to the huge increase in imported used car volumes, as a knock-on from the weakening of sterling after Brexit, has made the cost to trade-up more expensive and has slowed-down new car sales.
Mr Nolan said it is crucially important that used car values, and particularly diesels which represent 70% of newest used cars, remain high, as trade-in values will be key in ensuring that the country can deliver on improving its future carbon and air quality performance from transport.
"Stable trade-in values will be crucial in relation to our ability to renew the national fleet and increase the penetration of zero and lower emitting cars," Mr Nolan said.
"With all of this we remain confident that such new and nearly-new diesel used cars will continue to hold good values, as diesel remains the workhorse for business and those in rural Ireland," he added.