Potatoes were not seen during period (in Europe at any rate) until very late. As a result they are not seen in period cooking. If you really don't think you can go an entire meal without potatoes, you might like to consider parsnip as a period replacement.
A Brief History of Potatoes
Potatoes are indigenous to the South America, and it is far from unusual for people with Incan personas to not use the vegetable in their own dishes. Thanks to the Columbian Exchange, potatoes were introduced in the 1500's after the Spanish conquest of the Incan Empire.
They were most likely introduced to England by Sir Francis Drake around 1586. Potatoes happened to grow incredibly well in Ireland, and became the staple food of that region by 1650. It is ironic that a vegetable we typically associate with Ireland actually is American.
For more information: http://en.wikipedia.org/wiki/Potato