Obviously, given its name, Shepherd's Pie began as a popular dish amongst the farming class of Shepherds and Peasants (no surprise given the farm fresh ingredients and ease of preparation).
The true shining star of the dish is the main ingredient - lamb. While some recipes call for beef, I don't think you have tasted the true deliciousness that is Shepherd's Pie without the lamb. It just gives it that perfect flavor. And I like to up the ante even more, using Guinness instead of broth or stock. It's delish.