According to The Telegraph:
The Duchess of Cambridge…marked St Patrick’s Day in traditional fashion.
In the past the role was famously carried out by the Queen Mother…
At the start of the ceremony, William was one of the first to receive a shamrock from his wife and, following tradition, tucked it under the regimental star on the front of his cap.
Officers received their sprigs from the Duchess and she chatted for a while to each man in turn.
Last was Domhnall, the regiment’s Irish wolfhound mascot, who had his head stroked by Kate before she pinned some shamrock to his collar.