Above is a map of this Sunday's St. Patrick's Day Parade, easily the most significant such event in the U.S. (sorry, New York). There had been talk of shortening the traditional route due to safety concerns, but a federal judge nixed that earlier this week.
The show will go on per usual, commencing at Dot Ave and West Broadway, wending through the neighborhood like a pointed index finger, and ending at Dot Ave and Dorchester Street. As for taking the T to the parade, it's probably best to ride the Red Line to Andrew or South stations (buses will be running from the latter); and to not count on the train stopping at Broadway because of the crowds.
And, as for driving in Southie on Sunday, March 20, best to avoid it from about 11 a.m. to 4 p.m. Sláinte.