Final answer:
The boy who sang "The Lass of Aughrim" to Gretta is Michael Furey in James Joyce's "The Dead."
Step-by-step explanation:
The boy who used to sing "The Lass of Aughrim" to Gretta is Michael Furey from James Joyce's short story "The Dead," which is part of his collection Dubliners. Gretta Conroy reflects on her past love, Michael Furey, after hearing a rendition of the song that had special significance to their relationship. Although the provided text snippet does not directly relate to "The Dead" or the character Michael Furey, understanding Joyce's work requires recognizing the deep emotional significance that simple actions or songs can carry within relationships and memories.