The Duke of Cambridge enjoyed some craic as he presented sprigs of shamrock to officers at the barracks, in Beavers Lane, Hounslow , on Thursday (March 17).
He also attached a ceremonial sprig to the regimental mascot, a four-year-old Irish wolfhound called Domhnall, who is used to stealing the show.
The Duke is the first British Royal Colonel of the 1st Battalion Irish Guards, who moved into Hounslow last year .
His wife Kate Middleton, the Duchess of Cambridge, has performed the St Patrick's Day honours for the last few years, in a tradition dating back to 1901, but she was unable to join the Guards on this occcasion.
'A great day to be Irish'
Major Simon Nichols MBE, quartermaster of the Irish Guards, said: "It's a great day to be Irish, and we're very privileged to have a royal colonel, who we're delighted could be here to present us with our shamrocks.
"We've been delighted by the welcome we've had from the people of Hounslow since we arrived. Everyone we've met has given us a warm welcome, from the mayor to the local shopkeeper."
For the first time in five years, the Irish Guards were able to celebrate the patron saint's day as one, having previously been separated due to operational commitments around the globe.
All 450 serving soldiers, proudly displaying their St Patrick's blue plumes on their bearskin hats, were joined by the 150 regimental association members and the Army Cadets, affectionately known as the Mini Micks, on parade in the main square.
In a poignant moment during the visit, the Duke presented an Elizabeth Cross to the mother of Major Harry Shapland , who was killed in the Iraq War when his helicopter was accidentally shot down by a US fighter jet.
Following the formalities of the parade, Prince William took the time to chat to cadets, veterans and officers, including Major Edward Smyth-Osbourne, his brother Harry's former Army boss.
READ MORE: 10 facts about the Irish Guards
'Plenty of the black stuff flowing'
After posing for official photos, Prince William joined the Guards for lunch as the real celebrations got underway.
The Guards had already warmed up with Gunfire - tea laced with Irish whiskey - after a dawn performance of Reveille, and Army chiefs ensured the barracks was well stocked with Guinness.
Company Sergeant Major Carl Laverty said: "There will be lots of jigs and reels, lots for the families to do and plenty of the black stuff flowing."
Asked whether soldiers missed not having the duchess present this year, he said they were "ecstatic" the duke could be there, though he joked "I think the wives wanted to see what she (the duchess) would be wearing".
Drummer Michael McNamara, Domhnall's handler, said the regimental mascot was used to getting plenty of attention at occasions like this and the Changing of the Guard , which is currently performed by the Irish Guards.
"Domhnall leads the Guards out for the Changing of the Guard so he's used to all the attention. Everyone knows him here. He gets a little jumpy sometimes but he's quite good with the drills."
The Irish Guards are due to represent the UK at the Somme 100 centenary in France on July 1 this year and in September will head to Belize for two months to practise their jungle warfare skills.
Major Nichols said they were also working on a number of community programmes with Hounslow Council , including taking a group of disabled and disadvantaged children to see the Trooping the Colour ceremony for the Queen's birthday this June.