Irmelin is an opera in three acts with music by Frederick Delius. Composed between 1890 and 1892, it was his first opera, and was not premiered until 1953, nearly twenty years after his death. The libretto was by the composer, and weaves together folk-lore stories. In 1931 Delius made a new Irmelin prelude, using themes from the opera, and this has entered the concert and recording repertoire.
In the opera the heroine, Princess Irmelin, sits in her castle waiting for the dream lover promised to her by voices in the air. She infuriates her father the King by rejecting all her knightly suitors. Meanwhile Nils, a prince enslaved as a swineherd by the robber chief Rolf, breaks free and finds his way to Irmelin just after her enforced betrothal to one of the suitors. The opera ends with the lovers disappearing together into the forest.