“5. And we’ll see the return of Saw, Mon, and Bo-Katan. Forest Whitaker will reprise his role as rebel extremist Saw Gerrera. Filoni is resurrecting another character from The Clone Wars — Bo-Katan, sister of the fallen Duchess Satine Kryze — seen briefly in the trailer raising the Darksaber. And Genevieve O’Reilly is back as Mon Mothma. “You might see Mon Mothma get a little fiery for once,” Filoni said of the generally level-headed leader. “There’s a reason she’s the leader.””
Source : StarWars.com.
Genevieve O’Reilly played Mon Mothma in Revenge of the Sith (where all her scenes with dialogues were cut), Rogue One and Rebels Season 3.