Hillsborough Castle in County Down joined London, Edinburgh and Cardiff in firing a royal gun salute in honour of the Duke of Edinburgh's birthday.
Prince Philip received the good wishes of the Prime Minister as he spent his 92nd birthday in hospital.
Buckingham Palace described the Duke as "comfortable and in good spirits" following the exploratory operation on his abdomen.
Gun salutes were fired around the country in honour of the Duke's birthday, at Edinburgh Castle in Scotland, Cardiff and Hillsborough Castle in Co Down, Northern Ireland.
The King's Troop Royal Horse Artillery fired a 41-gun royal salute at noon to mark Philip's birthday.
The 21-gun royal salute was fired at Edinburgh Castle by officer cadets from City of Edinburgh Universities Officer Training Corps (UOTC), of which the Duke is Honorary Royal Colonel.
Commanding Officer of Edinburgh UOTC, Lieutenant Colonel Gordon Mackenzie, said the cadets were "thrilled" to be part of the pageantry in front of large crowds.
He said: "The officer cadets have just spent two weeks at their annual camp practising everything from survival techniques, building military bridges, and conducting peacekeeping operations in a fictional country.
"Firing a 21-gun salute for our Royal Colonel shows yet another aspect of what university students can experience in the UOTC."
Visitors to the castle also saw 1st Battalion Scots Guards on duty when they provided the Castle Guard. They wore their red tunics and bearskin headdress for the occasion.
The Duke was admitted to the London Clinic for the pre-arranged operation on Thursday, following a garden party at the Palace that day.
Analysis of the results of the operation are expected to be completed this week, and the Duke is likely to be in hospital for up to two weeks in total while he recovers.
Details of the original symptoms behind his admission have not been released, but it is thought the procedure did not involve his heart or bladder, organs the Duke has experienced problems with before.
It is believed that Philip has been visited on a daily basis by royal doctor Professor John Cunningham.