In "Danny Boy: The Legend of the Beloved Irish Melody," Malachy McCourt plays sleuth in tracing the origin of "Danny Boy," the most beloved of all Irish melodies. WGT Consulting Editor Rick Grant says McCourt's book creates a context for the song, albeit leaving its meaning as murky as ever. Oh, Danny boy, the pipes, the pipes are calling From glen to glen, and down the mountain side The summer's…