GUEST - it's originally Scottish (tune first known from 1625, words from about 1800, lots of later Scots versions of the words) but the versions most often sung are Irish adaptations, from the late 19th century. There are so many slight and not so slight variants that you can easily do people's heads in by picking an unfamiliar one.

The Scots title is "Goodnight and God be with you all", sometimes with "joy" instead of "God".