Wow they make floating widgets now !!
The ball contained CO2 which releases itself when the ring pull is pulled, to put all the little bubbles in the Kilkenny Irish Cream. This is so that you get that pub draft effect. Flat Kilkenny is not as nice as Kilkenny with all the gas bubbles.
Edit - Wikipedia has an explaination http://en.wikipedia.org/wiki/Widget_%28beer%29
The ball indeed contains Nitrogen.