Shane Ross (Dublin Rathdown, Independent) | Oireachtas source

We expect that by 2023, half of the vehicles will be low-emission varieties and by 2030, diesel vehicles will be gone completely. Buses will be replaced when they reach the end of their optimal life. For the double-deck fleet this is approximately 12 years and on this basis a steady state annual replacement need for Dublin Bus is 80 to 85 vehicles and for Bus Éireann this is 20 to 30. Additional vehicles may be also required to increase capacity on the network to cater for increasing travel demand or any new services.

See this speech in context