Now then, pay attention:
St. Patrick was some guy who was taken as a slave to Ireland and escaped home after many years and many unspecified adventures concerning, um, sheep and so on (oh just go look it up on Wikipedia). But he hated the damp and miserable country of his imprisonment so much that he decided to come back and fuck with their heads and convince them that Christianity was in some way better than the fornicating, wine-drinking and guilt-free Druidic way of life.
“Hmm, let’s see now…you’re telling me we get sin and guilt instead of dick and fun? You know what, you’ve convinced me!” What a salesman…let us all thank the Lord that he wasn’t a telemarketer.
So then he proceeded to drive “the snakes” out of Ireland. Of course, there never were any snakes so modern historians believe this was a reference to penises, as in “driving out the penis as well as all the fun the druids were having with them.” No more of THAT, thank you very much.
Having made Ireland a dry, guilty and worried place he then proceeded to introduce a foreign plant, the shamrock, in an attempt to crowd out and destroy the native leprechaun fungus (Lephrecatus edulis) a commonly used psychedelic, and the last source of free enjoyment on the island.
Finally, having sucked every last grain of fun from the country, he retired to Armagh, built himself a castle and laughed at the natives’ confused attempts to understand the Christian bible while altar boys polished his staff-shaped penis.
And, oh yeah, then he invented Guinness and the colour green.