It is the second consecutive month in which the figures showed an increase over the corresponding months last year.
However, numbers for the first five months still reflect a market in which car sales are falling.
The SIMI figures point to a 4.7% increase in new car registrations in May to 6,341.
That compares to 6,055 registrations in the same month in 2018.
However, new car registrations year to date are down 7.6% on the same period last year to 79,343.
The number of imported used cars continues to rise with 9,346 vehicles registered here in May, an increase of 4.7%.
Year to date imports are 3% ahead of 2018 at 45,062.
New electric vehicle registrations continue to grow month on month with a total of 1,902 electric cars registered so far this year, according to SIMI, surpassing the total of 1,233 EVs registered for 2018.
“Registrations for the month of May showed an improvement on May of last year, although the increase in numerical terms was relatively small,” Brian Cooke, SIMI Director General said.
“However, sales overall year to date have been disappointing with both business and consumer Brexit related uncertainty contributing to dampened demand for new vehicles. As we move into June the Industry is now focused on preparations for the start of the July 192 registration period.”