The company that took over dozens of routes from Dublin Bus last year is to be fined for letting the service slip.
Go Ahead will also be closely monitored, after passenger complaints about cancellations and delays around Dún Laoghaire.
The National Transport Authority confirmed the action in response to a request from a local TD.
Richard Boyd Barrett says he was told Go Ahead "is in breach of contractual obligations".
The company is said to be blaming a poorer service on a high number of drivers resigning.
It's believed they are being offered better pay and conditions to work for Dublin Bus.
Last year, Go Ahead took over the running of dozens of routes from Dublin Bus.
It agreed that services would remain the same, or improve in frequency.
However, there have been a rising number of complaints about services mostly in the Dún Laoghaire area.
They include the routes 17, 45a, 59, 63, 75, 111 & 114.
Passengers have reported the buses either turn up extremely late, or not at all.
The National Transport Authority said that Go Ahead is liable for fines after failing to meet its contractual obligations in June.
In a statement, it said "the delivery of certain bus services by Go Ahead during June was below the target set out in the contract".
However, the NTA went on to say that Go Ahead has recently been able to recruit and train new staff.
The NTA is "currently seeing an improvement" in Go Ahead's performance, but will continue to monitor it.
Ed Wills, Managing Director of Go-Ahead Ireland, said the company is "dedicated to improving public transport".
He said they want to "outperform the targets set out for us by the National Transport Authority".
He also promised "improvements to our timetables over the coming months".
Deputy Richard Boyd Barrett has described the privatisation of the routes as "a race to the bottom".
He has suggested the routes should be handed back to Dublin Bus.
Public transport operators are often fined for not meeting targets.
During 2018, Dublin Bus was forced to pay out €1.2m, while Bus Eireann was fined around €600,000.
