The shamrock is a symbol of the lucky Irish - but the Duchess of Cambridge had a spot of bother when she struggled to pin the sprig to an army mascot dog.
Kate, 33, presented the bunches of clovers to soldiers from the Irish Guards to mark St Patrick's Day today.
But as she bent down to pin the sprig on the silver collar of the regiment's beloved mascot, Irish wolfhound Domhnall, it kept falling off.
Quartermaster Major Paul Fagin had to step in to give the royal a hand at the ceremony in Mons Barracks, Aldershot, Hampshire.
The dog handler Drummer Dean Kirk said: "It wouldn't go on his collar, we had a bit of trouble with the shamrock. The shamrock fell off."
Kate later joked about the mishap with three children who presented her with a posy of white flowers.
She told eight year-old Ciara Flynn and her twin sisters Leah and Eva, both six, that it was tricky.
The girls' mother Mrs Flynn, 41, whose husband Mark is company sergeant major of the regiment, said: "She spoke about giving the dog the shamrock, and said it didn't stay on for very long."
The three year-old dog, who weighs 73kg, the equivalent of 11-and-a-half stone, was treated to a sip of Guinness after the ceremony.
Kate, who is due to give birth next month, beamed as she arrived at the ceremony with the Duke of Cambridge.
She covered her baby bump in a chic brown coat by Catherine Walker and wore a matching brown hat by Lock & Co.
Pinned to her coat was the gold shamrock brooch which was originally presented by the Irish Guards to the Queen Mother - who traditionally led the ceremony when she was alive.
William, 32, attended the traditional parade as Colonel of the Regiment and wore a navy blue frock coat.
The couple spent around an hour at the parade and chatting happily to soldiers and their families afterwards.
But Kate, who is eight months pregnant, joked with one mother that when she bends down she struggles to get back up.
Becks Cansdale and her nine week-old son Jonty met Kate after the ceremony. She said: "She asked whether Jonty was feeding and sleeping well, I said he is feeding but not so much sleeping - that's boys for you, I'm sure Prince George is the same.
"I gushed slightly and said you look amazing for eight months pregnant.
"She smiled and said she bends down and has trouble getting back up."
Cleeve Acheson, 96, who has 11 great grandchildren also met Kate.
The retired sergeant major said: "She remembered me from last year, she wished me happy St Patrick's Day.
"There are 11 in my family - I wasn't trying to drop a hint as to how many she should have, but it raised a laugh."
Three-year-old Alexandra Noel was wearing a pink sparkly dress when she met Kate.
Her other Cassandra said: "It was her dream to meet a princess and now she has met one."
The ceremony was first inaugurated in 1901 by Queen Alexandra, the wife of the then king, Edward VII.
Ever since then a senior female member of the royal family has carried it out.