Prince William's blunder comes after his brother, Prince Harry, made a similar faux pas at the Commonwealth Youth Reception last week, when he and Meghan appeared to enjoy a private joke.
Her choice of Givenchy is an unsurprising one since she has become an ardent champion of the French fashion house under Claire Waight Keller's creative direction in recent months, and famously wore her custom design on her wedding day.
Wife Camilla, 70, had to step in and assist the monarch who needed his gloves in place before walking out of the venue with the rest of the royal family.
Meghan Markle can be seen in the video above crossing her legs.
Meghan and Harry will head straight from the service to Dublin for a two-day tour.
HELLO! spoke to Leeanne Hundleby, the co-owner and founder of Strathberry who revealed: "We are delighted that the Duchess of Sussex chose to carry Strathberry for her début official oversea visit to Dublin, the third time she has championed our brand".
They will also visit the Book of Kells as well as Guinness Storehouse and the Dáil.
Harry and Meghan arrived at Aras an Uachtarain in the city's Phoenix Park at 9.30am.
The Duke and Duchess will also attend a summer garden party at the British Ambassador's residence, where they will meet people from across Ireland's arts, sports, military and social enterprise sectors.
The programme will include official meetings with The President of Ireland, Michael Higgins, and with The Taoiseach, Leo Varadkar.