The royal visitors descended into the garden after their expected arrival time of 3.30pm.
The slight delay meant that scores of the visitors were waiting intently to see the glamorous young couple. Many of the party guests were themselves young.
When the Duke and Duchess of Cambridge emerged from the house, they and their accompanying dignitaries – who included the host, the Secretary of State Theresa Villiers – stood on the terrace for the national anthem before descending into the garden party.
The couple then split as they were introduced to different guests.
Among the first people that Prince William was introduced to was the PSNI Chief Constable George Hamilton.
“He was very engaging and genuinely pleased to hear the stories from police officers and police staff that I had the honour of introducing him to.”
Those PSNI staff included the head of the Terrorism Investigations Unit, police constables, a finger print expert and a helicopter pilot.
“The prince and the helicopter pilot could have talked all day,” said Mr Hamilton. “He was very interested in policing.”
As they moved round the garden, the royal couple were particularly interested in one garden party guest by the name of Bob McAllister.
Mr McAllister, who is aged 91 and was a missionary for 40 years in the Congo, first told the Duchess the story of his 40 years in the African country until 1992.
“That’s absolutely extraordinary,” she could be heard saying to Mr McCallister, a Presbyterian.
He spoke about how his late wife had been a midwife looking after pygmy people in the Ituri forest.
Mr McAllister had lived in Congo when the Irish mercenary Mike Hoare was there and also lived under the rule of the dictator Mobutu.
Catherine was so intrigued by the story that she briefly left Mr McAllister to bring over Prince William so that he could also hear about the Congo.
Mr McAllister then revealed to the royal guests the remarkable detail that he was born on the very same date as the prince’s grandmother, Queen Elizabeth – April 21.
However, he was born one year earlier to the day. “I told him that the Queen’s birthday and mine was the same date. Every year there is a gun salute in London and I take a bow here in Northern Ireland,” laughed Mr McAllister.
The Second Word War veteran, who served in France and Germany, said yesterday’s meeting at the garden party was the first time that he had ever met a royal.