The National Transport Authority (NTA) has said it intends to use up to 600 double-deck hybrid buses as part of the BusConnects programme.
It has started a procurement process for the double-deck buses, which it says will be a Single Supplier Framework Agreement.
This is part of the transition to low emission buses, including electric buses, for urban public bus fleets.
The framework agreement would allow the NTA to buy double-deck diesel-electric hybrid buses from the supplier over a period of about five years.
The first delivery could take place before the end of 2020, the NTA says.
The buses will be double-deck vehicles with a minimum of 40 seats on the upper deck, and 17 on the lower deck.
This is not including so-called 'tip-up' seats.
They will also need a wide front entrance doorway, fitted with a twin-leaf door and a motorised ramp for wheelchair access.
There will also be a dedicated buggy space.
It adds that any bus proposed by a manufacturer in this tender will be expected to meet the definition of a 'clean vehicle'.
This is to meet the EU's revised directive on the promotion of clean and energy-efficient road transport vehicles, which is expected to enter into force until sometime in 2021.
An NTA spokesperson says: "The new hybrid buses are expected to be at least 30% more energy efficient than current diesel-only buses, and as a result will reduce harmful greenhouse gas emissions.
"The NTA also intend that the new buses will be capable of zero-tailpipe-emissions operation.
"This capability, combined with the overall reduction in energy consumption, will contribute to an improvement in air quality in the Irish cities that will be served by these buses."
The BusConnects network will create 230km of dedicated bus lanes and 200km of cycle lanes.