The show, which is being executive produced by Grey's Anatomy creator Shonda Rhimes, is set in 1895 and focuses on the first luxury hotel in New York.
O'Byrne has now signed up to play Edwin Lily in the project, Deadline reports.
Edwin decides to open the luxury hotel after a scandal hits his family.
O'Byrne recently starred as Reg Duffy in NBC's Prime Suspect, while his previous roles include parts in Mildred Pierce, FlashForward and Brotherhood.
Sarah Bolger has already signed up to appear in Gilded Lilys as Edwin's daughter Violet, a young woman who falls in love with a ship's servant despite being engaged to a wealthy family friend.
Blythe Danner and Madeline Zima have also landed roles in the pilot, which is being penned by Gossip Girl producer KJ Steinberg.