Modern history
In 1826 the Royal Engineers used the cairns to establish base stations on the summit for the Principal Triangulation of Ireland in advance of the Ordnance Survey six-inch-to-one-mile scale maps. The original construction of the Mourne Wall between 1904 and 1922 by the Belfast Water Commissioners to enclose the water catchment in the Mournes then added to changes to the structure of the Great Cairn.
The region was designated as an Area of Outstanding Natural Beauty in 1986, and the National Trust bought the 1,300acre site which includes Slieve Donard, Slieve Commedagh, Thomas Mountain, Millstone Mountain and Shan Slieve from the Annesley Estate in 1991. The Annesley family had owned the land since the 1700s. The National Trust continues to work to protect and preserve its history for future generations.