Gardaí in Co Wicklow have begun an investigation into a fire that broke out at Russborough House last night.
The alarm was raised at about 9pm, and as many as eight units of the fire brigade worked for some three hours to bring the fire under control.
The west wing of the building was extensively damaged and part of the roof collapsed.
The Alfred Beit Foundation said today no works of art were damaged by the fire.
"The whole west wing is being restored so everything had been taken out," said a spokesman. "The work is roughly a third of the way through, so was stripped down at the moment."
Initial examinations of the damage suggested an electrical fault from wiring in the roof may have sparked the fire.
A Hollywood film crew led by Oscar-winning film director Steven Soderbergh had finished shooting scenes at the stately home about 48 hours before the fire broke out. Knockout, which stars Ewan McGregor and Michael Douglas, is filming in Ireland for three weeks.
The 18th-century State-owned property in Blessington - which has an extensive art collection - was recently included in a list of endangered heritage sites alongside the likes of Machu Pichu in Peru and the Sagrada Familia in Barcelona.
The house was designed by Richard Cassells and was donated to the State in 1978 by owners Sir Alfred and Lady Beit. The house is open to the public and is managed by the Alfred Beit Foundation.
The mansion, built in 1741, has been targeted in several notorious robberies.
In 1974 an IRA gang, including British heiress Rose Dugdale, stole 19 paintings valued at €10 million after tying up Sir Alfred Beit and a member of staff.
In 1986, Dublin criminal Martin Cahill, known as the General, stole €38 million of paintings, and in 2001, robbers rammed a car through the front door of the house and stole two more paintings. Sir Alfred and Lady Clementine continued to live there until their deaths in 1994 and 2006.
It attracts one million visitors a year, and staff said the fire damage would not affect summer opening times from May onwards.
Russborough is home to much of the original Beit Collection, a substantial selection of paintings returned in April 2008, having been in the National Gallery for the previous six years. The refurbishment of the west wing was due to be complete by Easter this year with management hoping to offer it for short term holiday lets.
Additional reporting PA