Personally I can't stand the Southie St. Pat's parade. Too many families and Irish Catholic thug/punks who revel in the fact that the parade bans gay participation (such as a float). Not that an activity has to have open gay participation to be enjoyed, but the ban does give you a clue as to what kind of event it is.
ANYWAY... Tons of pub crawls, shit like that if you get into that kind of thing. Check this guide out:http://www.boston.com/thingstodo/special/stpatricksday/
I could offer more if you gave me an idea of what nights you'll be around and what types of things you get into.