While Herbert does describe Shaddam as looking 35, despite being 70, I think it is hard for an actor only 35 to truly pull off the gravitas of a character of that age and station. We also need to remember that people are already aging much slower than they did during Herbert's generation.
With that in mind, Ewan McGregor is my pick. He looks much younger than his age, he is great in villainous roles, and he can pull off that presence. Fassbender and Bale both look a little rough for their ages due to their body transformations for roles.
Pugh is completely wrong for Irulan. Irulan is described as tall, thin, blonde, fair, with a haughty air to her. She is meant to be a physical contrast to short, elfin, and down to earth Chani, who is practical and less regal.
Elizabeth Debicki is basically perfect for Irulan. She is supposed to older than Paul and Chani as well.