She was an aristocrat and an eccentric who greeted guests with a cockatoo on her shoulder.
Lady Mairi Bury of Mount Stewart, County Down, who died last year, was a colourful character who piloted her first plane aged 11 and her last at 85.
She was a close friend of politicians like Harold Macmillan. She was also one of the UK's greatest stamp collectors.
Her collection is expected to raise £2.6m at auction in Sotheby's, London, on 24 - 26 November.
Sotheby's have described it as one of the finest collections of British stamps to come on to the market in the past 25 years.
The auction house said it would need three days to sell off 2,185 lots.
Sotheby's philatelic consultant Richard Ashton told the Guardian: "She was fascinating because although there would have been very little that was outside her financial reach, she was far more interested in buying something that added to the interest of her collection.
"Something for £50 gave her as much pleasure as something that was £5,000."
The collection is also unusual because there are very few stamp collectors who are women.
Lady Mairi started collecting stamps at an early age; later in life she formed important collections of Cook Islands, the Falkland Islands and Hong Kong.
Great Britain became the focus of her collecting activities.
Over the course of her collecting career, Lady Mairi had several philatelic advisers who would always be searching for some gem to add to the collection.
Mr Ashton said Lady Mairi had a good eye for rarity, quality and the unusual. She had developed a love of cancellations, particularly if there was an association with her beloved homeland.
In the collection are fine examples of the penny black and two-pence blue.
She also owned a rare printed envelope telling the story of the iron-paddle steamer, Nemesis, used in the first opium war of the 1840s by the East India Company.
The only other recorded example is owned by the Queen.
The first lot in the auction is not a stamp but a parliamentary envelope. It dates from the days when MPs could send and receive a certain number of letters as long as they autographed the corner.
This one is special, because the corner is signed by one Robert Peel.
Mr Ashton met Lady Mairi several times.
"You'd have your first sherry by 10.30am," he said. "She was eccentric, but in the nicest possible way.
"She would wander around with a parrot on her shoulder - everything about her was different and people were absolutely devoted to her."
Ladty Mairi was the youngest daughter of the 7th Marquess of Londonderry and lived most of her life at Mounstewart which she gave to the National Trust in 1977.
She was an accomplished photographer, a great shot, a skilled fisherwoman and stalker and a qualified mechanic who suprised her daughters with miniature tool kits at Christmas when all they wanted was a new dress.
She continued to live in a private apartment at Mountstewart until her death at the age of 88.