New car sales in July were up over 60% on the figure for the same month last year, marking a strong start to the 142 registration period.
According to motoring group SIMI, 18,776 cars were sold in July - a 61% increase on the 11,620 cars sold in July 2013, the first year of the dual registration plate.
It makes July 2014 the best July on record for vehicle registrations.
So far this year, 84,474 new cars have been sold, an increase of 30% on 2013.
It's already 10,000 more cars than were sold in the whole of last year.
"We have to be positive about these figures, which are even stronger than we expected," Alan Nolan, Director General SIMI said.
"When SIMI proposed the introduction of the split registration system, we predicted that it would give consumers and businesses a better choice in buying their new car mid-year rather than during the busy first quarter."
Heavy goods vehicle registrations are also up 40% for the month of July and 44% year to date, the figures show.