The precise origin of Irish whiskey is unclear, though believed to have originated among the monasteries where a great deal of mathematics and science was preserved throughout the medieval and Renaissance periods. While most distilleries suffered through periods of temperance in British culture, a few were able to survive, particularly as an export to England.

The potato on the other hand, came to Europe from the New World. Simply put, by that time, Irish grain whiskey was already established as an domestic and export product. It would have been unwise to change over to a drastically different product.

For clarification using potatoes doesn't define vodka. Vodka is just an odorless clear liquor produced from any fermented base. It's produced using a reflux still that essentially scrubs the impurities from the vapors resulting in vodka.