Their efforts have been supported by police, Coastguard, Mourne Rescue Team, Forestry Service, National Trust, NIEA and Sky Watch Patrol.
Coastguard helicopters from the Republic of Ireland and Great Britain allowed fire chiefs an aerial view of the blaze to help inform tactics, transport personnel to remote locations and plan resources.
The public has been urged to stay away.
Aidan Jennings, assistant chief fire and rescue officer, said it is “undoubtedly one of the most challenging gorse fires firefighters have ever had to deal with”.
Aidan Jennings asked people to be extra fire aware (Rebecca Black/PA)
He said fire crews will be available to those who need them despite the operation in the Mournes, but asked the public to be extra fire aware.
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