I've been told that for Ireland St. Patrick's Day is a national holiday so the streets are littered with passed out bodies. My friends that are Irish here though didn't feel that the level of partying was even worth the effort so they actually did less than most of the other students. Other things I learned: saying St. Patty's day is not OK but St. Paddy Day is.
Wearing green for the day is not a part of the culture here. I'm guessing it's because of the fight over Northern Ireland and the British side is orange for that fight. That being said an excuse to drink is not passed up so St. Patrick's Day is heavily celebrated here. Of course since St. Patrick is the saint for Ireland the drink of the day is Guinness. The big traditional thing to wear here for St. Patrick's Day is the hat. Now normally I am told that you are given the hat when buying four pints of Guinness. However this year, as long as you donated two pounds to the charity you got the hat. Which I was happy about since I wouldn't have got the hat otherwise. My friend however had it all planned out a little to well of which chains of pubs gave which kind of hats (Yes, pubs are franchises here like McDonald's which dictates which ales are served.) and how in previous years that he had drunk 8 pints to get both kinds of hats (The Guinness top hat, and a clover leprechaun hat.)