The 109x bus from Cavan to Dublin received over 200 complaints per 10,000 passengers last year.
The National Transport Authority published its complaints report for 2018 that highlighted the number of complaints against each of its routes.
Quarter 2 saw the highest number of complaints on the Cavan route, with 74 per 10,000 passengers.
Across the entire year, there were 264 complaints per 10,000.
Over 12 thousand were made to the operator last year, according to the figures from the National Transport Authority.
Two-thirds of those complaints relate to service experience with staff issues making up 1 in every 10 complaints.