How does James Joyce manage to convey a sense of futility, the theme of an unlived life, in his short story "The Dead"?

The themes of futility and an unlived life are most clearly expressed in the final scene of the story, when Gretta and Gabriel have a conversation about a boy she loved when she was very young. She is reminded of him when Bartell D'Arcy sings a traditional song, "The Lass of Aughrim." The final lines of the song are "My babe lies cold within my arms but none will ever know." When Gretta says she is reminded of someone she knew who used to sing this song, Gabriel misunderstands and becomes jealous. But Gretta relates that the young man Michael Furey died long ago, at the age of seventeen, and she believes he died because of her.

As Gretta tells this story, Gabriel holds her hand, but she is unresponsive. He recalls holding her first letter to him that same morning. This image conveys a sense of longing and unrequited love: Gabriel's thoughts of Gretta are literally in his hands, but she "did not respond to his touch." Greta continues to tell him the story of Michael Furey: of how he came to see her the night before she was to go away for a few weeks. He was already ill and exposed himself to the cold rain in order to see her. He died three weeks later and Gretta was inconsolable. She cries when she tells Gabriel of this:

Gabriel held her hand for a moment longer, irresolutely, and then, shy of intruding on her grief, let it fall gently and walked quietly to the window.

The story is told from Gabriel's vantage point, and so the reader also gets a sense of being somewhat outside of Gretta's grief, and being shocked to realize she has carried it all these years, and it has been a part of their marriage. He looks on her as she sleeps, as if he has not fully known her until this moment, and "as he thought of what she must have been then, in that time of her first girlish beauty, a strange, friendly pity for her entered his soul."

The feeling of futility is palpable here, as Gabriel feels helpless to help Gretta in her grief, and also feels distant from her, as if their marriage has been based upon a secret he has only just learned. He is overcome with emotion for his wife, but at the same time feels a sense of loss for the things about her he doesn't know and may never truly understand: 

He thought of how she who lay beside him had locked in her heart for so many years that image of her lover's eyes when he had told her that he did not wish to live.

He then lets his thoughts wander to his aunts Julia and Kate, and how they may well be dead soon. This thought fills him with regret: "One by one, they were all becoming shades." He considers Michael Furey again, and considers him lucky to have died young and not to have withered with age. In the final scene he looks out the window at the snow falling and imagines it falling on the landscape, as well as "upon all the living and the dead."