Maggie Lemass was a young Irish lady, sister to Sean, who caught the eye of Indiana Jones during his visit to Dublin, Ireland in 1916. With her best friend, Nuala, she was interested in gossip, candies and clothes.
Indy first saw her and Nuala through the glass of a patisserie shop and sat with them. Maggie believed Jones to be an "American millionaire." At first, Jones encouraged the idea, but after a while he realized Maggie mainly enjoyed Nuala's company while Jones only would pay for their meals and entertainment. Once he brought Remy Baudouin along, in order to distract Nuala, and Jones was then free for a brief courting.
Eventually her brother, Sean, a Fenian who was trying to recruit Sean O'Casey, discovered that Jones was no more than a waiter at Rooney's pub, earning money in order to leave for London. During a political disagreement between the two men, Sean was against their relationship and threatened Jones to end it.
Eventually, after spending much money for her and not enjoying her company, Indy decided to end it and revealed who he was. Maggie proclaimed that she never wanted to see him again.
However, they were reunited when, against Maggie's dissuasion, Sean barricaded himself in the post office along with other Fenians as part of an Easter Rebellion. Maggie went to Rooney's to ask Indy for his help; when troops of British cavalry arrived to respond to the situation and during the conflict, she stayed in the pub with them.
In the aftermath, Jones escorted Maggie to the prison where Sean was detained.