Go-Ahead has apologised for buses in Dublin either not arriving or turning up late - but it says its latest figures will show complaints have halved.
The firm currently operates more than 20 routes in Dublin.
Figures for April to May found grievances more than doubled on some routes since the company took over last year - with a total of 16 of the routes seeing a rise in complaints.
Three routes - the 104, 114 and 76A - each received more than 100 complaints per 100,000 customers in Q2 2019.
Meanwhile, almost half of services failed to meet punctuality targets during the same period - with the company facing fines for failing to meet their overall 65% punctuality target.
Go-Ahead managing director Ed Wills has insisted the company is constantly improving.
He said: "If there's complaints for the general [...] service, for that I apologise - our aim is to deliver the best service we possibly can.
"I think a lot of what we experience, certainly in the stages of our mobilisation... were people contacting us having concerns about a lot of the times of their buses changing.
"We had a lot of comments coming in about the changes to capacities on the vehicles."
Meanwhile, People Before Profit TD Richard Boyd Barrett says privatisation of services should be scrapped.
He suggested: "We have a lot of the new Go-Ahead services in the greater Dun Laoghaire area, and since those routes started up we have had consistent complaints about buses not turning up or not turning up on time.
"It really just shows that the privatisation and outsourcing of routes is a misguided strategy, and it should be abandoned."