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.
Firefighting operation at Mourne Mountains being scaled back | Prestwich and Whitefield Guide
prestwichandwhitefieldguide.co.uk - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from prestwichandwhitefieldguide.co.uk Daily Mail and Mail on Sunday newspapers.
Firefighters return to tackle Mourne Mountains blaze for third day
nwemail.co.uk - get the latest breaking news, showbiz & celebrity photos, sport news & rumours, viral videos and top stories from nwemail.co.uk Daily Mail and Mail on Sunday newspapers.