Kate, 37, got in the holiday spirit with a forest green Alexander McQueen coat that she topped with a matching fascinator.
The Duchess kept her hair pulled back and accessorized with a pair of green tourmaline and green amethyst Kiki McDonough earrings.
She also added a shamrock pendant to her left shoulder and completed the look with black gloves and heels.
The Telegraph reports the brooch is a royal heirloom by Cartier and previously belonged to Queen Elizabeth the Queen Mother.
Last year, Kate participated in the traditional toast — but since she was pregnant with Prince Louis at the time, she stuck to a glass of water while Prince William enjoyed a few sips of Guinness.
This year, both royals indulged in the traditional pint, posing as they sipped the suds.