Word of advice on flights. Flying into Atlanta will probably be your cheapest route, but it's a four hour drive of the most boring stretch of highway in the South. Rt. 16 from Macon to Savannah has few exits and is trapped heavily. If it doesn't break the bank, fly directly into Savannah.
We just had a hockey tournament in Savannah this past weekend and a room at a decent hotel with an easy walk to River or Bay St. was going from $200-$250/nt. Figure at least that for St. Patty's Day...probably more. Staying out by the airport is cheaper, but not as fun. Can be walked in a drunken stupor, but I wouldn't recommend it. Staying in the middle of the party allows you and your buds to come and go as you please. I've had a St. Patty's day cut short because we were with the girlfriend's friend who can't hang and she needs to get back to the hotel to crash and doesn't feel safe by herself. Started drinking at 10am and she's down for the count by 8pm. C'mon???!!! Waited for the girls to pass out and we headed back out. That was 6 years ago and I still catch stojan for it.
I'm not a huge fan of tours, but outside of St. Patty's activities I'd recommend a haunted pub crawl. Good way to find out of the way places to grab a drink. You'd be amazed at the number of bars/pubs/restaraunts that are in old homes and buildings. Without the pub crawl, I wouldn't have even realized they were there.
What else....Moon River is a decent microbrewery and just don't hit the bars on River St. There's a few good Irish, Speakeasy (Crystal Beer Parlor on Jones St.) and Colonial style (Six Pence on Bull St.) pubs a few blocks off.
Grab a sandwich at B.Matthew's Bakery/Eatery on E. Bay. They have some kind of sandwich made from black-eyed peas or chick peas (can't remember) that is fantastic. This coming from a guy who has to have meat with every meal.