Here are the Dubliners singing this bittersweet ballad. Luke Kelly also sings the song. By my lights, the Clancy Brothers do it best.
This old folk song is also called The Brave Grenadier. A soldier courts his love, and they "kiss so sweet and comforting" as they cling together. She asks if he will marry her, only to be told he has a wife in his own country. Even so, they enjoy the song of the nightingale together, as they sit by a spring.
As recorded for the British Library, the real bird sounds like this.