"Danny Boy" is a popular ballad written by English songwriter Frederic Weatherly and usually set to the Irish tune of the "Londonderry Air". It is most closely associated with Irish communities but interestingly Weatherly himself never set foot in Ireland.
This piece comes from Xerxes - an opera seria in three acts. The opening aria, "Ombra mai fu", sung by Xerxes to a tree, is set to one of Handel's best-known melodies, and is often played in an orchestral arrangement, known as Handel's "Largo". It is often performed at solemn occasions such as funerals and weddings.