The following is from Wikipedia, but this is such a wonderful song too by Peter Mayer, "Blue Boat Home". I highly recommend going to church to sing to the song with 200-500 people.
Hyfrydol (Welsh pronunciation: [həvˈrədɔl], meaning 'cheerful') is a Welsh hymn tune which appears in a number of Christian hymnals in various arrangements. Composed by Rowland Prichard in 1844, it was originally published in the composer's handbook to the Lutheran Hymnal Cyfaill y Cantorion ("The Singers' Friend"). Prichard composed this hymn tune before he was twenty years old.