The National Trust has completed the purchase of Divis and the Black Mountain, which overlook Belfast.
The uplands, which rise to more than 1,500 feet, have been closed to public access for decades because much of the land was in private ownership and because of the location of British military transmitters.
Under a £2.9 million (€4.1 million) scheme, control of the scenic hills transfers to the trust, the UK equivalent of An Taisce.
A programme of investment covering paths and trails provision, the documentation of items of value to heritage as well as transport and tourism initiatives will now begin.
The National Trust has been helped in the purchase by the Environment and Heritage Service of the Department of the Environment, and by the Heritage Lottery Fund.
The area comprises some 1,500 acres of Divis and virtually all of neighbouring Black Mountain which had been held by the Ministry of Defence.
It has significant archaeological value, with evidence of Neolithic and prehistoric settlements, including hut circles and monuments.
It is also home to numerous species of birds and wildlife, with slopes covered by heathers, mosses and bog.
The trust owns and manages some 40 square miles of scenic lands and maintains them for public use.
These include some 60 miles of coastline including the Giant's Causeway in Co Antrim.
It also owns Slieve Donard in the Mourne Mountains and a range of estates and homes.
Some of the trust's 40,000 Northern Ireland members will assist voluntary schemes to upgrade paths and signage in the Belfast hills before full public access is opened by next summer.
Speaking in Belfast Ms Ruth Laird, the local director of the National Trust, said: "Today marks a new chapter in the history of the trust in Northern Ireland.
"We can now begin to preserve and protect the rich and varied wildlife on the mountains and, working with the Belfast Hills Partnership and local communities, open up a landscape that affords spectacular views of the city, Belfast Lough and much of Northern Ireland."
The National Trust's director-general, Ms Fiona Reynolds, said: "We delight in the opportunity that this acquisition gives us to contribute in such a significant way to the people of Belfast and further afield."
Mr Kevin Baird from the Heritage Lottery Fund said the area was a wonderful heritage asset.