Incumbent Eamon Ryan is Minister for Climate Action and Transport but faces a challenge from his deputy Catherine Martin.
The Greens helped form a new three-way coalition Government in Dublin last month and have forced measures to combat global warming and sustainable transport into the spotlight.
The Government has passed the #NORABill, unlocking €500m for the Climate Action Fund over the next ten years to support innovative climate change solutions.🌎
This fund will create jobs, reduce fuel poverty, improve air quality and biodiversity and build resilient communities. pic.twitter.com/eUbPAWKT3j— Green Party Ireland (@greenparty_ie) July 22, 2020
The two candidates have held their party roles since 2011.
They are divided over whether experience is essential for the leadership role, while Ms Martin wants to promote more women to ministerial positions.
Ballots in the leadership contest were issued to 2,336 party members in the Republic and 598 members in Northern Ireland.
The Green Party recorded its best ever general election result in February, winning 12 seats – an improvement of 10 from 2016.
It has entered into Government with larger rivals Fianna Fail and Fine Gael, who buried their differences in an historic realignment of politics to face a left-leaning opposition led by Sinn Fein.
Among the pledges of the new Government is achieving an average 7%-a-year reduction in overall greenhouse gas emissions from 2021 to 2030, representing a 51% reduction over the decade, with the aim of achieving net zero emissions by 2050.
It envisages a Green New Deal of retrofitting thousands of homes, and pledges an accelerated electrification of the transport system including electric bikes, electric vehicles and electric public transport alongside a ban on new registrations of petrol and diesel cars from 2030.
More public funds would be invested in walking, cycling and public transport and there would be a major drive to realise the potential of off-shore renewable energy.
Counting of ballots in the leadership contest will be overseen by independent auditors while observers from both candidates will be present.
Ms Martin told a previous hustings that experience was not a “prerequisite” to becoming a leader.
After the Greens voted to go into Government, Ms Martin said that there was a “missed opportunity” by her party to promote women within its ranks to ministerial positions.
Ms Martin said those who opposed the Programme for Government document were excluded from the Cabinet table.
Mr Ryan’s pitch focused on his experience in government and as minister for climate action.
He attracted some criticism when he appeared to be sleeping during a vote in the Dail last week but it is not expected to impact the final vote.
Once the vote has been declared, both candidates are expected to give short speeches.
