Officials at Hillsborough Castle say they have been left "shocked and saddened" at the sudden death of a staff member who played a central role in preparing the royal residence for visits.
Dorothy Arbuthnot (nee Jess), died suddenly at the family farm, understood to be outside Lisburn, on Saturday.
The popular grandmother worked at Hillsborough Castle for more than six years as part of the household team.
She assisted in the recent visit of Prince Harry to the Secretary of State's annual Garden Party in September.
In a death notice, she was described as the loved wife of Tom, mother of Gary, mother-in-law to Susan, and grandmother to Kyle and Lois.
Patricia Corbett, Head of Hillsborough Castle, Historic Royal Palaces, said: "We have been deeply shocked and saddened by the untimely passing of our dear friend Dorothy Arbuthnot.
"Dorothy joined Hillsborough Castle in January 2012 as part of the household team.
"Over the course of the six years that have followed, Dorothy became part of the lifeblood of the Castle, showing an unerring passion for her job. Her dedication to her role was exemplary.
"She served as part of the team that helped with arrangements for important events such as royal visits and visits by the Secretary of State, as well as providing valuable support for the day-to-day operations at the Castle.
"A truly irreplaceable character, Dorothy will be sorely missed, and our thoughts and condolences are with her family and loved ones at this time."
The castle has been a royal residence and the home of the Northern Ireland Secretary since the 1970s. Peter Cardwell, a Special Advisor at the Northern Ireland Office, was among those to pay tribute to Mrs Arbuthnot following her death.
Writing on his Facebook page, he said: "Dorothy was the kindest, loveliest, hardest-working person you could ever come across.
"Through my work at the NIO I sometimes had the privilege of staying at Hillsborough Castle overnight.
"Dorothy was always there to make sure everything was just right, that I was fed and watered, and helped with anything I or the Secretary of State needed.
"When Prince Harry came to the Secretary of State's Garden Party in September, it was extremely wet and muddy.
"I was due to meet the Prince a little later in the afternoon. Dorothy took one look at my muddy trousers and chuckled 'you can't meet the Prince looking like that, Peter'.
"Typically kind, she took me down to the laundry and took a damp tea towel to my trousers, which looked perfect when the time came to meet Prince Harry.
"Recently, I decided to write to the CEO of Dorothy's employer to praise her outstanding work and kindness to me.
"I have lost a friend."