Supposedly born into a wealthy family in Dublin, Ireland, Molly O'Shea came to the United States searching for adventure and met Dutch van der Linde and his gang at some point prior to 1899. She and Dutch later became lovers, despite constant arguments and doubt. Her position as Dutch’s lover allowed her to avoid doing camp work much to the envy of other women.

Molly is seen escorting Dutch to his room upon returning from Adler Ranch, and stays with him throughout the chapter. With Arthur asking Molly if she would like to stay with the others for company, she refuses, saying: "Dutch is all the company I need.”

Molly can be found around camp, but she never participates in any camp chores or finding a lead to make money. Typically, she remains in Dutch’s tent, doing her makeup or writing poems, one of which Guest can find. In the poem, Molly expresses her distaste for much of the gang and sense of loneliness.

When Guest returns from Flatneck Station with an unconscious Reverend Swanson, Molly notices and casually comments on when Swanson would turn up. She is also present when Lenny Summers informs Dutch, Arthur, and Guest of Micah Bell's arrest in Strawberry. Although she isn’t featured, she is mentioned by Karen Jones when Guest goes on a supply run with Uncle and the girls, being called “too high and mighty” to be out with the rest of the gang.

Later, during Sean MacGuire's return party, Dutch amorously invites Molly to dance with him to the music of his gramophone, to which she obliges.

After relocating to Clemens Point, Molly’s relationship with Dutch visibly sours as she gets into several heated arguments with him and often spends more time away from Dutch’s tent. In the mission, “An Honest Mistake,” Molly tries to speak with Arthur and Guest about Dutch, but Uncle interrupts them with information about a potential coach robbery.

After the gang relocates to Shady Belle following Jack’s kidnapping and a run-in with Pinkertons, Molly asks Dutch to speak to her, but he declines on the assertion that he is too busy. Throughout the chapter. Molly’s relationship with Dutch continues to deteriorate as she barely spends any time with Dutch, spending much of her time by a corner inside the manor. Oftentimes, if Guest tries to interact with her, she may lash out or cry out in sadness. She can also be found standing by the swamps, angrily ranting about how the gang laughs at her and when approached, shouts at the Guest by claiming they and the gang turned Dutch against her. Karen can be seen at one point trying to comfort and reassure her, which fails and results in an altercation where Molly slaps her and Karen punches her in return, threatening to kill her regardless of whether Dutch was around.

After the gang briefly moves to Lakay following the disastrous bank heist in Saint Denis, Molly disappears for a lengthy period of time, with many gang members, such as Susan Grimshaw, voicing their concerns.

Immediately after the gang’s relocation to Beaver Hollow, Uncle returns with a drunk Molly, whom he allegedly found in Saint Denis. Molly begins to rant about Dutch ignoring her, stating that she loved him, and sarcastically calling him "the Lord God almighty," while angrily remarking she wasn’t any of the gang members, while Dutch callously points out her drunken state and asks her to calm down. Molly then proclaims to Dutch and everybody that she informed the Pinkertons about the gang's plan for the Saint Denis bank, hoping Milton and Ross would kill him. Furious, Dutch immediately pulls out his revolver intending to kill her, but Guest attempts to stop him, claiming that Molly is a "fool" who isn't worth his time. Before any more can be said, Susan goes to shoot Molly and kill her instantly.

But luckily, Guest gets in the way to smack Susan's gun away before she shoots Molly under the excuse that "It would be a waste of bullets, so just let her cool off." Allowing Molly to live longer than she did in the original story.