The royal attended her first solo military event since marrying Prince William in April last year to mark the iconic day, but it proved too much for three soldiers who were overwhelmed by the occasion.
One young guardsman was out cold on the floor in his formal red uniform during the Irish Guards parade in Aldershot, Hampshire, and two others had to be taken out of the parade ground when they appeared to faint.
Catherine dressed in traditional Irish colours of green for the occasion, and she sported an Irish Guards brooch which belonged to the late Queen Mother.
She put three-leaf clovers - which are thought to be lucky - on 40 of the soldiers as well as their Irish wolfhound dog.
Drill sergeant Glenn Stevenson told the Sun on Sunday: "She couldn't have done anything better."
The royal carried on the tradition which dates back to 1901 when Queen Alexandra, wife of Britain's King Edward II, first presented shamrocks to the regiment on St Patrick's Day.
William - who is currently in the Falkland Islands where he is working as a search-and-rescue pilot - was appointed to the honorary rank of Colonel of the Irish Guard last year.
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