Final answer:
The moment Gretta hears 'The Lass of Aughrim' sung by Bartell D'Arcy is climactic as it reveals her emotional past, prompting significant changes in the characters' inner lives and advancing the plot towards its resolution.
Step-by-step explanation:
The scene in which Gretta hears Bartell D'Arcy singing 'The Lass of Aughrim' represents the climax of the story as it brings to the forefront deep emotional undercurrents and triggers the unfolding of central themes. The song evokes a poignant reaction from Gretta, revealing her connection to a past love and setting the stage for the protagonist's self-realization and epiphany. This moment is climactic because it marks a turning point in the narrative, dramatically altering the characters' inner lives and serving as the catalyst for the subsequent falling action and resolution of the plot. The intensity of the emotional response, the connection to past events, and the changes it precipitates all underscore why this scene is the narrative's climactic point.