750 electric vehicles will be delivering post around the country by 2022.
200 e-trikes and electric vans will be making zero emission deliveries by the end of the year.
An Post is also pledging zero emission deliveries in Dublin city centre by the end of 2019.
The pledge covers the areas between the Royal and Grand canals in the capital.
Meanwhile, the environmentally-friendly service will be rolled out in Cork, Galway, Kilkenny, Limerick and Waterford next year.
Other initiatives being rolled out by the company include a trial for public charging points for electric vehicles outside post offices.
"We're moving away from mail"
An Post CEO, David McRedmond, explained that the volume of parcels being delivered means regular bikes are being phased out.
He observed: "Most of what we'll be doing in the city centres will be electric vehicles... vans.
"The reason why we need them is because we're going to move away from bicycles - we're moving away from mail, and being an e-commerce parcel company.
"An Post is all about delivering parcels."
Climate Action and Environment Minister said An Post is 'taking the lead' in responding to climate change.
750 recharable vans and e-trikes will be delivering our post in Dublin, Cork, Limerick, Kilkenny, Galway and Waterford by 2022.
It’s part of An Post’s new eco plan pic.twitter.com/CweQ0YGxMp
— Kacey O'Riordan (@KaceyORiordan) February 20, 2019
He observed: "The public service and our semi-state bodies must be the first to show that it takes policies for sustainability seriously, if we are to persuade the rest of society to make the step changes which we need to make.
"By placing sustainability at the core of their business strategy, [An Post is] leading by example."
It's claimed the new An Post initiatives will save 1,000 tonnes of carbon annually.