(Since so little actually happens in this play, here's the very short list of things Mary actually does. If you want a timeline of how her character develops through conversation, which is more important to a play like this, head to the "Detailed Summary.")
Mary sits down and chats after breakfast with the family.
She goes upstairs to "nap" (read: take morphine).
She comes downstairs for lunch; it's obvious that she's high as she talks to Edmund and Jamie before lunch.
After lunch, she yells at James for a bit, then heads upstairs for another hit.
She comes back down soon after, even more zoned out, and speaks with James.
After going into town to get her morphine, she sits in the living room chatting with Cathleen. Once Cathleen leaves, she sits by herself for a bit.
Mary decides to take another hit, but the return of James and Edmund stops her.
After speaking with both of them, she goes upstairs to take more morphine instead of having dinner with her husband.
Finally, at the end of the play, Mary comes down with her wedding dress to find Jamie, Edmund, and James seated in the living room. She has the last word.