Elba plays Duncan, general of the royal army and in charge of training the young troops.
Galitzine appears as He-Man, with Camila Mendes, Jared Leto and Alison Brie also appearing.
O’Riordan, who is from Upminster in east London, said she was “ecstatic” to win the role of young Dian, a member of the royal guard trained with He-Man as a child.
“She kind of bullies him at school, so it was quite fun to play that sort of role, because I haven’t played something quite mean [before],” she said.
She described the stunt training as “really fun”, adding that she found some synergy with her co-star.
“The older version of me, Christiaan (Bettridge) – she’s trained in ballet and she’s also a stunt woman.
“So, when she was doing the stunt, she was really elegant and I’m trained in ballet as well. So people say that we had very similar styles in the combat.”