You can’t mature Irish whiskey in a metal vat. You can’t age it in your bathtub. You can’t use plastic or glass or surgical steel in Irish whiskey maturation. In order to be designated as such, Irish whiskey must be matured for at least three years in wooden casks no larger than 700 liters. But there’s no rule that says those casks can’t be previously used.
So if a cask that once held American bourbon goes on to age Jameson, then gets borrowed by an Irish brewery up the street to age a stout, then returns to Jameson to finish more whiskey — that’s totally acceptable by law, and thoroughly enjoyable by the glass.
Caskmates Stout edition is Jameson whiskey, finished in casks that were used to age stout at Franciscan Well, an Irish brewery in Cork. The flavor of the finished whiskey is classic Jameson, but with notes imparted by the stout that include dark chocolate, coffee, and butterscotch. Learn more at Jameson.