The Irish Waterfall is the first
smoking 'trick' anybody ever learns. I use inverted commas because it's not much of a
trick. It's ridiculously easy to perform. Those of you who have never done this could probably
blow waterfalls with the best of them after reading this, even if you've never smoked in your life.
The basic effect is thick white smoke rising up one's nose. You will have seen Snoop Dogg do this in pretty much every rap video he has been in. The point is for the smoke to permeate your nasal walls and reach your blood, and thus your brain, quicker.
Basically when you take a drag of the joint (pot is a far more appropriate herb for this 'trick' than tobacco; tobacco smoke is too dark and harsh) do not inhale. This is the basis for other smoking tricks such as smoke rings and variants on rings. Now slowly open your mouth so that your lips are just barely parted, and push your jaw out slightly so your lower lip is further out than your upper lip. This gives the smoke a channel to flow out of. Do not exhale now, just let the smoke rise of itself. It must be smooth and fluid, a thick white sheet of smoke rather than a puffy cloud or thin lines of smoke. The smoke must be opaque.
Now inhale thru your nose. Simulataneously let the smoke out of your mouth while sucking it up your nose. Again, this must be smooth. If you're doing it right, this should look quite sexy and cool and it should get you stoned almost instantly.
Practise a little to get it just right. Like with smoking a cigar, this can be quite unfamiliar and therefore difficult to pick up but after a few tries you will be fine.
Also (more popularly) known as the french inhale. In the real world I have only ever heard the term 'irish waterfall' in reference to this act, but more people on the internet seem to know of it as a french inhale. There will probably be some kind of reason behind this.