An Irish Waterfall is whilst smoking you inhale the smoke and then open your mouth, whilst doing this the smoke will rise up out of your mouth. As the smoke is rising out you inhale back through your nose which will the take the smoke from your mouth back into you throat and lungs recycling it.
Snoop Dogg is seen quite often hittin an Irish Waterfall.
Its hard to execute and when its done right its of tha hook!