The pipeline forms a key element of Irish Water's €144 million Cork Lower Harbour Main Drainage Project.
Once in place, the pipeline will pump wastewater from Cobh to the new Shanbally wastewater treatment plant for safe discharge to the lower harbour.
The water utility, in partnership with Cork County Council, is working to end the decades-long practice of discharging raw sewage into the harbour.
Two pipes will be installed under the River Lee estuary between Cobh Dockyard and Monkstown as part of the estuary crossing contract.
The pipes will be installed using a horizontal directional drilling construction technique to reduce disruption to residents, businesses and road users and with the least environmental impact.
Once the Cobh networks contract is complete in 2021, all of Cobh town’s wastewater will be connected to the Shanbally plant by the estuary crossing pipeline for treatment before its discharge to the harbour.
Since the project started, the amount of raw sewage discharging untreated into the harbour has halved — from the equivalent of 40,000 wheelie bins every day to 20,000 — through the construction of the Shanbally plant, and connection of Crosshaven, Carrigaline, and Shanbally in December 2016.
Ringaskiddy was connected to the treatment plant in October 2018, and wastewater from Passage West, Glenbrook and Monkstown were connected this summer.
"However, as part of this operation, some additional survey works may be required from time to time using boats and or divers along the pipeline route," they said.
Upgrading Cobh's wastewater pipe network to connect to the new pipeline will result in street closures in the town.
The West Beach and East Beach road between the former Permanent TSB branch as far as The Wellhouse has closed to vehicle traffic with the closure remaining in place until November 30.
A section of the R624 road at Rushbrooke will also closed this week and will remain shut to cars until November 25.