The tender process to buy 600 double-deck hybrid buses for the National Transport Authority has begun.
The first of the new fleet should arrive by the end of 2020.
The new hybrids will serve urban and suburban routes in Ireland's main cities. The NTA wants all 600 buses delivered by one supplier over the next five years.
As a condition of the tender they must all comply with the EU's clean vehicle directive which comes into effect in 2021.
Each bus will have a low-floor section with a wide front entrance door suitable for unaided wheelchair access.
They will also have a dedicated wheelchair space, a dedicated buggy space and a wide centre exit doorway.
They are also expected to have an efficient operational life of at least 12 years.
The new hybrid fleet will be 30% more energy efficient than the current diesel-only buses and the NTA says this will help to improve the air quality in the cities they serve.