Mary is not working when we first meet her. She is on strike over a trade union dispute about a "clear case of victimisation" against a fellow worker. She is trying to get a better life for the working class.
Like her brother she says "a principle’s a principle", but in contrast to Johnny we see her actually standing up for her beliefs by striking in protest.
As well as coming across as a militant trade unionist in the opening scenes of the play, she also seems an idealist when it comes to romantic relationships.
She is disappointed in her romanticised view of the world when Jerry Devine rejects her because of her pregnancy. She tells him poignantly, "your humanity is just as narrow as the humanity of the others".
While initially an idealist, Mary has learned from experience that people are not as brave and compassionate as she had imagined.
Despite her initially naïve view of the world, Mary is portrayed as an intelligent girl.
She is involved with Jerry Devine and Charles Bentham - both educated men - and she speaks knowledgably about politics and current affairs.
Boyle himself mentions that she is reading Ibsen at one point. Later he regrets that she should have intellectual aspirations like these when he says, "What did th’ likes of her, born in a tenement house, want with readin’? Her readin’s afther bringin’ her to a nice pass."
Intelligence and a desire to better oneself are not met with admiration in Boyle’s world.
Mary talks about going to Jerry’s "lecture in the Socialist Rooms some time ago, on Humanity’s Strife with Nature" and remembers many of the verses.
She recites its poignant words, showing that there is both beauty and horror in the world.
There is a tragic conclusion to Mary’s story, she is most likely going to suffer the indignities that would befall a single mother during that time in Irish society.
For despite her curious, bright nature there is - as shown in Boyle’s condemnation of her reading - no place for women like her.