"My mother was born and raised in England and always made Shepherd's Pie with beef. I've heard the lamb, beef argument before so did a little research: Cottage pie refers to a British or Irish meat pie made with beef mince and with a crust made from mashed potato. A variation on this dish is known as shepherd's pie.
The term cottage pie is known to have been in use in 1791 when the potato was being introduced as an edible crop affordable for the poor "cottage" meaning a modest dwelling for rural workers.
In early cook books, the dish was a means of using leftover roasted meat of any kind, and the pie dish was lined with mashed potato as well as having a mashed potato crust on top.
The term "shepherd's pie" did not appear until the 1870s and since then it has been used synonymously with "cottage pie", regardless of whether the principal ingredient was beef or mutton. Enough said. "