Her Royal Highness also presented 106-year-old veteran, Norman Irwin, with a BEM for his voluntary work in his hometown of Coleraine.
Norman joined up at the outbreak of the Second World War serving throughout the war and is the last living founding member of the Corps of Royal Electrical and Mechanical Engineers.
These photos show some of the atmosphere of the special day.
9. Garden party
Princess Anne pictured at the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye
10. Garden party
The Princess Royal attends the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye
11. Garden party
Guests at the garden party at Hillsborough Castle enjoy the experience despite the weather. Photo: Jonathan Porter / Press Eye
12. Garden party
Princess Anne plants a tree during her visit to Hillsborough Castle. Photo: Jonathan Porter / Press Eye