Westport House has been sold to a local business family, while Mayo County Council have acquired 40 acres of the estate.
The house was bought by the well-known local family Hughes family of Portwest and The Hughes Group.
The news will be confirmed at a press conference in the estate today.
Ganly Waters oversaw the sale of the 10-bedroom property by Nama, which was on the market for €10m, making it the nation's second most expensive country home for sale this year.
Previous owners Browne family today confirmed the sale of the historic property and said that estate administrators have agreed the terms of the sale of the estate and house to Hotel Westport.
The owners of Hotel Westport will invest €50m in the project and will create 200 jobs for the local area.
"This is an emotional day for our family," Ms Sheelyn Browne said today.
"On the one hand, we are handing over custody of our ancestral family home after hundreds of years but we are doing so in the knowledge that the new owners are committed to bringing to fruition the ambitions and dreams of our late and much loved father Jeremy Browne.
"We are indebted to the estate administrators Con Casey and Dermot Furey for their unrelenting efforts to secure the future of the estate.
"We also want to thank all those who supported our efforts from our local TDs to the community of Westport. In particular we would like to thank Minister Michael Ring, Peter Hynes, Chief Executive of Mayo County Council and his team and NAMA for working constructively and collaboratively with our advisers.
"There is one group that deserves special mention. That is our hard-working and dedicated small team at Westport House. We would not have been able to survive the last three difficult years without their unwavering support and loyalty. They know who they are and we as a family owe them a great debt of gratitude."
The family also said they were "touched very deeply" by the support shown by the locals in Westport over the "last few difficult years".
Meanwhile, Chairman of the Hotel Westport Mr. Cathal Hughes said they plan to make the multi-million euro investment and create jobs within the next five years.
"We plan to invest up to €50m in new facilities which will lead to the creation of 200 new jobs over the next five years," Mr Hughes said.
"On behalf of the Hughes family I want to wish the Browne family every success in the future and I look forward to liaising with them as we develop our plans over the next few years.
"I want to assure all the existing staff, suppliers and customers that we will continue to operate as normal under the new ownership."
It has also been claimed as Ireland's most beautiful.
Whether or not this is the case, the ancestral home of the Marquesses of Sligo with 455ac certainly leaves you breathless at the richness and quality of each and every main room inside, and is therefore one of the few abodes to truly qualify for that much bandied term: 'stunning'
The entrance hall is somewhat famous as the only surviving Richard Cassels-designed interior of a Doric Frieze and coffered barrel-vaulted ceiling.
Its inner end opens to an imperial staircase of Sicilian marble which was designed by George Wilkinson and has balustrade details by Francis Skidmore, the supremo of 19th century metal work.
Almost every top craftsman in these islands through the late 18th and 19th century has had some hand here. Each room is a show-stopper, among them the Chinese bedroom, which features a full wall-to-ceiling hand painted depiction of the famous willow pattern, telling the tale of a tragic love story between a young oriental couple.
The original house was built in the 1650s by Colonel John Browne on the foundation of Grace O'Malley's Castle. He married O'Malley's great great granddaughter, Maude Burke.
The current house dates largely from the 18th century and stands as a two storey block with nine bay front and two bay wings on either side.
At 38,210 sq ft, it would accommodate the floor space upstairs and downstairs of almost 40 standard city homes.
Ganly Waters also completed the sale of former Health Minister James Reilly's mansion Loughton House this week.
Westport House in numbers: