Irelia Masteries Help!

I like Irelia a lot, but I see a lot of different masteries for her. I want to know what masteries to pursue.

I have seen 21/9, 9/21, and 13/17.

I don't understand 21/9 because all the percentage increases don't apply to her true damage, which is her main damage output. Yet 9/21 feels too weak at times. I would like the community's opinion on the matter so I can make an informed decision.