When crews from Kilkeel arrived, the fire was already through the roof, a spokesperson said.
Firefighters battled to save historical artefacts from the property. However, many were lost in the blaze.
John Allen, Fire and Rescue Service Area Commander, confirmed many valuable items were removed from the house, including paintings and antique furniture, which were saved before the roof on the main part of the building collapsed.
He said afterwards: “This is one of the largest residences anywhere in the Mourne area and it’s an absolutely beautiful building.”
Adding that it was sad to see such a stunning home in such a state, Mr Allen emphasised that he was grateful much of the remainder of the building could be saved.
The former Northern Ireland Office minister Sir Richard Needham said he was devastated at the weekend’s events.
Sir Richard, the 6th Earl of Kilmorey, said he has fond memories of the house, which was rebuilt in 1806.
The damage was so extensive that several floors collapsed, leaving the property currently unstable.
The earliest parts of the estate date back to the 1550s.
Mourne Councillor Harold McKee visited the estate over the weekend and on Monday paid tribute to the work of the firefighters.
Environment Minister Alex Attwood expressed his concern at the damage caused to the historic stately home.
He described the fire as “bad news” and “a sad day for heritage”.
“This listed property is a great example of the scale of our built heritage. The Department of the Environment has worked with the property owners to preserve and restore the property in the past.
“I have asked for an early report on what we could do now,” the minister said.
Meanwhile, South Down MP Margaret Ritchie said she was shocked at the news and wanted to ensure all efforts are made to assist in the restoration of the “historic property.”