May saw 9,581 cars taxed for the first time, a drop of 5.4 per cent compared with the same month last year.
Meanwhile, 7,732 second-hand cars were imported in May, an increase of 40.9 per cent on Mar 2016.
While the number of cars being taxed dips steadily from January to June and then spikes again in July, the number of cars taxed in the first five months of the year was still down 10.3 per cent compared with the same period last year.
Similarly, the number of cars imported in the first five months of the year was up by 49.6 per cent on the same period in 2016.
The most popular make of new cars taxed in May was Volkswagen, followed by Renault, Nissan and Toyota. Those four brands represent 36.6 per cent of all cars taxed in the month.
In terms of imported cars, Volkswagen was the most popular brand in May, followed by Ford, Audi, and Toyota.
The least popular new car taxed in May was Ssangyong, while it was a tie in the import category between Opel and Ssangyong.
