British firm GoAhead has been announced as the preferred bidder for six bus routes on the Kildare-Dublin commuter route.
It means Bus Éireann will no longer service those routes.
Last year, GoAhead was also selected as preferred bidder for 10% of Dublin Bus routes.
Today's decision means GoAhead will also operate the 120, 120C, 123, 124, 126 and 130.
Those services link Dublin city centre to towns such as Athy, Kildare, Maynooth and Naas, as well as Edenderry, Tullamore, Portlaoise and Enfield.
The National Transport Authority (NTA) is promising a seamless transition between operators from passengers' perspectives, although says there will be some timetable changes to 'optimise the services'.
However, the authority says that fares will stay the same, and Chief Executive Anne Graham stressed the services are not being privatised.
She explained: "Nothing is being told, services are not being deregulated. The control of the services remains with the NTA.
"The operator will be paid a fixed fee for providing the prescribed services - and the fares go to the NTA, not the operator."
The changes are due to come into effect at the end of this year or early next year.
GoAhead is one of the largest bus operators in London. It is also one of the two partners in Govia, the parent company that runs British railway lines such as the Gatwick Express and Southern.
'Today's news is disappointing'
The chief executive of Bus Éireann, meanwhile, expressed his disappointment that they were unsuccessful in their bid for the Kildare-Dublin routes.
Ray Hernan said that while the news was difficult for staff and management, there is also cause for optimism.
He suggested: "While today's news is disappointing for everyone at the company, I believe we must now focus on delivering even higher standards of service on the 240 routes we continue to serve, to ensure we protect and grow our position for the future.
"While it has been a challenging year with significant change for all of our staff, this news indicates that while it was heartening to win our first competitive tender last November against four other bidders - we must now strive to be even more competitive, not just on cost, but on the quality of our service delivery."
He added that he wished GoAhead "well in their endeavours".
Bus Éireann was announced as the preferred bidder for five bus routes in Waterford city late last year.