Lol. It's cool dude. Nothing dodgey.
It's when you blow smoke out of your mouth and breathe it up your nose. Looks like an upside down waterfall.
It's called an 'Irish' waterfall as a play on the 'stupid' or 'backward' Irish thing.
That's called a French Inhale...I've never even heard the term "Irish Waterfall" before.
__________________ Fingerbang:
1.) The sexual act where a finger is inserted into the vagina or anus. Headbang:
1.) To vigorously nod your head up and down.