There are three distinct dishes known as 'Cumberland Pie' - the traditional Cumberland Game Pie dating back to the 18th Century, and intermediate form of the early 20th Century as the Cumberland Potato Pastry Egg and Bacon Pie, leading to the modern potato-topped Cumberland (Potato) Pie.
The traditional Cumberland Game Pie is now rarely made and modern 'Cumberland Pie' is an enriched variant of Shepherd's Pie or Cottage Pie. Minced beef and vegetables (typically onion and carrot) under a mashed vegetable crust, most commonly potato, with grated cheese (sometimes with breadcrumb), baked to crisp on top. This current form seems to have appeared as a cook-chill ready-meal around 1990.
Several other dishes have borne the name, including:
Double-crust pie of apples, currants and lemon (Clutha Leader, New Zealand, 26 October 1888)
Onions, parsley and cold lamb slices under slices of raw potato, baked. (Leader-Post Newspaper, Regina, Saskatchewan, Canada; Mar 14, 1947)
Custard tart made with added butter and nutmeg (Florence Times, Alabama, USA; April 23, 1967)