It’s believed that Sir Walter Raleigh introduced the potato (a New World crop) to Ireland in 1589. Barley was obviously being grown there much earlier, although I can’t find a citation on when cultivation began, The word whiskey comes from the Gaelic uisce beatha—water of life. The possibly apocryphal legend is that monks had learned distilling techniques in the Mediterranean in the 11th century and brought those to Ireland. The earliest Irish whiskey reference is from 1405, and the Scotland started distilling later that century, Whether barley (or other grains) were being distilled earlier than that is a matter of conjecture.

Edit: a good mainstream resource on this topic is A History of the World in 6 Glasses by Tom Standage.

Edit: listed the right book.