They will attend a commemorative service, followed by a flypast including up to 100 aircraft.
This is only the second time Markle has worn colour since her May 19 wedding to Prince Harry, the first being at a reception last Thursday, for which she chose a yellow Brandon Maxwell dress. William was the fourth successive generation of the monarchy to become an RAF pilot.
Harry and Meghan fly to Dublin as part of a series of royal visits that the Royal family have planned in the wake of the Brexit vote, as they look to cement ties with European neighbours ahead of Britain's departure from the EU next year.
The Duchess of Sussex changed into a black midi dress for a summer party held at Glencairn House in Dublin.More news: Mahrez signs five-year deal with Man City
Meghan, meanwhile, went for a darker look in a retro-style, bespoke Dior dress featuring her go-to bateau neckline.
Two outfits in one day!
The duchess accessorised with nude heels, a silvery fascinator with a bow and floral accents, and the prestigious Air Cadets Dacre brooch, which is the highest award a female cadet can receive.
Kate Middleton and Meghan Markle joined the princes for a service at Westminster Abbey.More news: Eden Hazard admits ‘dreaming’ for Real Madrid deal, restates Chelsea loyalty
The newlyweds have a series of engagements lined up over the next 48 hours.
Harry and Meghan will "meet the people of Dublin and visit organisations which are central to Irish life".
According to Kensington Palace, "They are looking forward to learning more about Ireland's history and experiencing its rich culture, as well as meeting the people who are shaping the country's future".More news: Aunt Meghan: Duchess of Sussex stuns in green at Prince Louis’ christening