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.
1. Garden party
The Princess Royal meets guests at the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye
2. Garden party
The Princess Royal attends the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye
3. Garden party
The Princess Royal meets guests at the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye
4. Garden party
The Princess Royal meets guests at the garden party at Hillsborough Castle. Photo: Jonathan Porter / Press Eye