He does so in a large main hall, but can only make hamburgers. During a rainstorm, Dorothy Stonewall (Ruth Coleman) and her parents (Mary Forbes and Lionel Pape), seek shelter in the castle, meet Danny there, and are invited to dine with him...on hamburgers. They find hamburgers to be a delicious novelty, and the inspired Danny establishes Ireland's first fast-food operation (in a castle, at least), and soon he has a thriving operation . His band members from New York show up, and provide music for the customers while Danny Sings---and there is no doubt that this IS Ireland's first castle-housed fast-food operation providing live music.
Mavouneen has taken a liking to Danny, but gets jealous when she thinks he is paying too much attention to Dorothy and decides to sell the castle, and this is not good news to the band members who are doing a thriving business selling white-castle hamburgers. So they contrive to reunite Danny and Mavourneen.