Last month 4,399 new 191-C cares were registered in the county. This is a drop of 655 on the number of new cars registered in January 2018.
The drop in sales in Cork was repeated nationwide with an overall fall 12.6% for new car registrations.
The Society of the Irish Motor Industry (SIMI) published the figures which also show a fall in new Light Commercial vehicle registrations which are down 16.3% (5,650) compared to January last year (6,753).
Heavy Goods Vehicle registrations are also down 11.4% (356) in comparison to January 2018 (402).
SIMI said the motor industry faced a significant and uncertain year with monthly declines in new car registrations figures, reflecting the substantial impact of Brexit.
Used car imports in 2018 reached the highest level on record at 100,755 accounting for 44.5% of the total car market in 2018.
Used car imports for January 2019 (9,006) have seen a very slight decrease 0.6% on January 2018 (9,061).
"Sterling weakness and Brexit related uncertainty were the key drivers in 2018 and have continued to be a characteristic of January 2019," SIMI said.
"The dampening of new sales volumes has affected both the car and commercial sectors and until the 29th of March [Brexit] this deep uncertainty will continue."
Electric vehicle registrations have increased significantly with 811 registrations in January 2019, which is more than for the whole of 2017 and over 60% of the total for the whole of last year.