A 150-year-old tunnel is being replaced in Dublin to ensure drinking water for over 200,000 people.
The water is supplied to residents in north Wicklow and south Dublin.
Irish Water has turned the sod on the construction of a new pipeline from the Vartry Water Treatment Plant to Callowhill.
Planning permission for the €29m project was granted in February 2017.
The construction contract was awarded to Roadbridge following a competitive procurement process.
The Vartry Water Supply Scheme was originally constructed in the 1860s and was a feat of engineering for its time.
The scheme included a four kilometre long tunnel under Callowhill to deliver water to north Wicklow and south Dublin.
It includes two reservoirs on the Vartry (upper and lower), a water treatment plant comprising primarily of slow sand filters, and 40km of trunk mains that deliver water to open storage reservoirs at Stillorgan in Dublin.
Irish Water says the tunnel is now in a poor state of repair and is being replaced by the new pipeline which will "secure and safeguard the water supply for the communities it serves which is one of the most densely populated areas in the country."
In February 2017, Wicklow County Council granted planning permission for the construction of the new water pipeline and additional structures.
The proposed works will include a new pumping station at the existing Vartry Water Treatment Plant, and four kilometres of buried pipeline from Vartry to Callowhill and a break pressure tank at Callowhill.
Attending the event, Minister of State Andrew Doyle said: "The coming together of Wicklow County Council, Dublin City Council and Dún Laoghaire Rathdown County Council at this event today highlights the importance of a single public utility to manage our water services.
"Having a strategic view of the requirements and investment that will allow growth of homes and businesses is vital to our social and economic success.
"The vision that built Vartry 150 years ago is echoed now as we continue to invest in the future."
British Pathé footage from 1923 shows when the Vartry Water Supply Scheme was extended to include the Upper Reservoir.