If you find a ping pong ball in Kilkenny can, don't worry. It is inserted into the cans by the beer makers. When I first heard the sound of an object in my can of Kilkenny, I got worried. I wondered if it was some machine parts that dropped by accident into the can during manufacturing.
Then I did a search and voila - the object (a 3cm plastic ball) was inserted in for Kilkenny and Guinness beer cans so as to manage the characteristics of the beer's head, i.e. the foam that adds to the creaminess of the beer. See the small white ball thru the can hole in the picture below? The ball is nitrogenated and releases nitrogen into the beer when the can is opened. The nitrogen helps to form smaller bubbles and triggered likewise reaction for the rest of the beer.
So next time if you find a small plastic ball in Kilkenny cans of beer, don't worry - it is meant to enhance your drinking experience!