Kate, dressed in an ivory dress and matching hat, and William were joined by William's father Prince Charles and his wife Camilla, along with younger brother Prince Harry and his wife Meghan, Duchess of Sussex.
The royal family ― sans Queen Elizabeth II and Prince Philip ― showed up for Prince Louis Arthur Charles' christening on Monday. And, she will be hosting President Donald Trump this Friday at Windsor Castle.
The Archbishop of Canterbury Justin Welby welcomed the royals at the entrance to the chapel.
"They are rewarding the loyalty of their really close friends and keeping the godparents from the tight, inner circle", one source said, commenting on the fact that some of Prince Louis' godparents have direct ties to some other royal godparents.
Markle's first look for the Irish trip is posh, simple, and will definitely please her Irish counterparts-who we know Queen Elizabeth has had some trouble with (her first ever diplomatic visit was in 2011).
Following the 40-minute service, The Duke and Duchess of Cambridge gave a private tea at Clarence House.
The 3-year-old looked angelic as she walked in holding hands with her dad, Prince William, and carrying what appeared to be a program for the church service.
The Duke and Duchess of Cambridge chose hymns O Jesus, I Have Promised and Lord of All Hopefulness for the ceremony.
Earlier today, members of the royal family traveled to The Chapel Royal at St. James' Palace for Prince Louis' christening.
Harry Aubrey-Fletcher: He went to Ludgrove School and Eton with William and his father is the Lord Lieutenant of Buckinghamshire.
They were served slices of cake taken from a tier of William and Catherine's wedding cake, the Palace said.