The work will include the building of a new bedroom wing and a ballroom.
They will also enhance the 18-hole golf course, golf club, and 800 acres of parkland.
The hotel has, from yesterday, temporarily closed ahead of the planned refurbishments which will begin in March.
The planned works are set to take approximately 18 months.
General manager Anita Higgins, said: “This is a new beginning in Adare Manor’s grand history, we are taking an intermission before starting a glorious new chapter in 2017.
“Great care and attention has been taken in drawing up the proposed plans which will ensure the renovations honour the building’s architectural heritage and maintain the hotel’s signature style and world renowned status.
“These upgrades are essential to modernising the facilities of the hotel and golf resort, as identified by an extensive economic, physical, and operations review of the manor.”
She said it is estimated the resort contributes in excess of €5m to the local economy annually.
“Great care will be taken during the refurbishment to ensure the architectural beauty of the 19th-century manor is retained. It is envisaged the newly refurbished property will increase visitor numbers, from both Ireland and around the world, to Co Limerick and the Mid-West.”
Mr McManus paid a reputed €30m to acquire the property.
Work will also include upgrading and restoring internal finishes, furniture and woodwork, and repairing the external stonework and windows. New designs by architects Reardon Smith, and interior designers Richmond, will enhance all guest bedrooms and public areas.
The hotel said the renovations will honour the building’s architectural heritage as a neo-Gothic masterpiece and embody Adare Manor’s signature style, while incorporating contemporary luxury and technological innovations.
A new additional bedroom wing will add extra bedrooms bringing the total room count to 104 bedrooms, while a new ballroom will have the capacity to seat 350 people, catering for events and international conferences.
The enhancements in the Manor House will also include a new state-of- the-art spa, pool and relaxation area, boardroom and cinema.