NI Water is leading the work, using helicopters to transport materials and local stone masons to rebuild the granite structure – which covers 15 peaks in the mountain range – with expert advice from the Mourne Heritage Trust.

The wall was ordered to be built by the Belfast City and District Water Commissioners in 1904 to define a 9,000-acre catchment area for the construction of the Silent Valley reservoir. Built without concrete, the task took 18 years.

Much of the damage to the wall has been caused by lightning strikes over the years.

NI Water said the first phase of the work will get under way this month with the restoration of a 2.5km section between Slieve Loughshannagh and Slieve Meelmore.

Matthew Bushby of the Mourne Heritage Trust said the project was a vital opportunity to address erosion on popular mountain walkways.

“We’re delighted to be involved, in particular we’re very keen to see a holistic project undertaken to repair the wall,” he said.

“As a body, we’re very experienced in upland park erosion control work and heathland repair work.

“We’ve been utilising helicopters for lifting materials such as stones and turf to these sites. The initial cost is high, but the effectiveness is massive.”

Consultant archaeologist Eoin Halpin said the project was bringing history back to life.

“From 1904 to 1922 it was built a metre thick and three high in places,” he explained.

“The physical involvement of producing that… the mind boggles. They’re using helicopters today, but in 1904 these guys were setting out at 6.15 in the morning for a day’s work.

“The men who did this have taken on almost mythical quality.

“You see a road scheme these days and you have diggers and trucks, but they did it with shovels, spades and wheelbarrows.

“It will be very interesting to see how a modern engineering company approach it in 2017.”

The 1981 book The Dam Builders by WH Carson features an interview with Annalong man Johnny Cousins, who worked on the original wall.

Living in farmhouses or camping on the mountainside and fuelled by soda bread, the workers would often face a three-hour climb before the day’s work of splitting and lifting granite slabs even began.

Mr Cousins said that after three months on the mountains the “stone men” were hard as steel.

“They were men with powerful shoulders and hands like shovels,” he recalled.

Two serious accidents were noted at the building of the wall over Bernagh.

A man called Ned Hagan broke his leg in a stone quarry, while Willie McVeigh lost a foot after it was crushed by a granite boulder.