Security arrangements at Russborough House, Co Wicklow, are once more under review after the fourth theft of art work from its €40 million-plus Beit collection.
Five pictures were taken by raiders early yesterday including a painting and a sketch by Rubens.
Gardaí said the gang used a stolen four-wheel-drive vehicle to ram a downstairs window through which they entered the house.
The window led directly to a room, known as the Saloon, in which the five works had been hanging.
Supt John Murphy of Baltinglass Garda station, who is leading the investigation, said the raiders "were in and out within a couple of minutes".
Two of the pictures, Rubens's Portrait of A Dominican Monk, and Adrien Van Ostade's The Adoration of the Shepherds (The Nativity), both of which were painted on panels, were removed from their frames for ease of escape.
The other three works, Venus Supplicating Jupiter, a sketch by Rubens; Calm Sea by Wilem Van Dervelde the Younger; and The Corn Field by Jacob Van Ruisdael, all of which were on canvas, were taken in their frames.
On entering the house shortly after 6 a.m., the raiders set off a burglar alarm which alerted gardaí to the scene. Shortly afterwards, the black four-wheel drive vehicle used in the raid - which had been stolen on Saturday from Terenure, Dublin - was found in a field about a mile away from the house.
Lady Beit, who normally lives there, was not in the house at the time.
The Garda said it did not wish to comment on the adequacy or otherwise of security arrangements at the house. The house's administrator, Ms Deirdre Rowsome, said it had a "highly sophisticated system" in place, which was "considered very good by the gardaí".
She said: "What can you do? You go on having more and more sophisticated alarms and systems, but they don't seem to deter them."
She added the motive of the robbery was unclear given "nobody could really put paintings of this calibre on the open market".
Supt Murphy agreed there did not appear to be a market for such paintings. However, he said, "obviously, there is some interest in them".
One avenue of inquiry being followed is that the gang stole the pictures merely to enhance their reputation among criminal peers. Former thefts at the house were organised by Martin Cahill, the criminal known as the General, and the Provisional IRA.
Lady Beit and her late husband, Sir Alfred Beit, a former British MP, bought Russborough in 1952 and placed it and its contents under a public trust. More than 40 paintings from their collection, which is insured for an estimated €40 million but is said to be priceless, were stolen in raids in 1974, 1986 and 2001.
Many were subsequently recovered in Ireland, England and Turkey. Only last week, two of the paintings stolen from the house last year were recovered by officers from the Arts and Antiquities Unit of the National Bureau of Criminal Investigation.
A man was arrested and is facing charges in relation to the robbery.
Last month, Head of A Man by Rubens, which was taken in the 1986 Cahill raid, was found in a house on Dublin's north side following a tip-off to gardaí.
Ms Rowsome noted "the paintings don't belong to a private collector. They belong to the people, and they're being stolen from the people."
Visitors to the house were turned away yesterday as the scene was being preserved for technical examination. It is, however, due to reopen to the public today.
