Shepherd's wife offers a breakfast of bacon and beans if he will come home; he refuses, he must tend his sheep. She offers a dinner of pudding and beef, then a supper of bread and cheese. Finally she offers clean sheets and a pretty lass. He accepts.
This seems to exist in two forms, "O Shepherd O Shepherd" and "The Shepherd's Wife." The two have identical plots, but the latter -- at least as recorded by Gordeanna McCulloch, based on the version in Herd -- *feels* much bawdier, as well as more fun. (Anne Gilchrist thinks it may be derived from a singing game, and it does have rather that feel.)
The distinction is so strong that I thought of calling them separate songs, but I can't imagine a clear dividing line.
The tune of the "O Shepherd O Shepherd" versions is described as a "modal version of Greensleeves." This is a bit strong; the tune has been altered in more ways than the simple removal of accidentals. - RBW