Flynn’s dime begins as a symbol of Mairin’s connection with the outside world, the proof she has of the life she led before she entered the Home of the Good Shepherd. The dime also comes to symbolize the emotional support Mairin believes she might be able to find when she leaves. 

The Mercury dime was a 10-cent coin minted in the US between 1916 and 1945. The figure depicted Lady Liberty with wings, but this image was commonly if erroneously identified as the Roman god Mercury, which led to the nickname. When Flynn first gives Mairin the dime, it’s to thank her for looking after Fiona at work and sharing her wages. He tells Mairin she can always call him for help— payphone calls in the 1960s were generally 10 cents, though rates could vary—confirming that he feels as protective of Mairin as he would of his sister. 

When Mairin is given her uniform in the Good Shepherd, she slips Flynn’s dime into her shoe as a reminder of her old identity, which she feels is being taken away. The dime also represents the hope that someone on the outside will believe and support her, which her mother is not offering at this time. When she sees Flynn’s grocery truck during their drive back to the Home from Niagara Falls, this serves to reinforce Mairin’s longing for freedom. After she escapes from the bookmobile and finds herself alone in Buffalo, the only money she has is Flynn’s dime. She is then able to use the dime for the purpose Flynn intended: Calling for help when she needs it.