From Jameson's website:
When the Jameson Master Distiller met the Master Brewer from Franciscan Wells for a drink, a plan was hatched to send a few Jameson casks to their brewery down the road for them to age their craft Irish stout. The time came for the casks to return home and our Master Distiller saw an opportunity which piqued his curiosity. He re-filled the stout-seasoned barrels with Jameson Whiskey, and Jameson Caskmates emerged; a whiskey that retains all the triple distilled smoothness of Jameson Original, with a truly unique finish. The subtle suggestion of hops, cocoa and coffee are signposts that it has bumped shoulders with stout; however, Caskmates is undeniably a member of the Jameson family.