Since the corporate faux pub Firkin & Beaver closed, Tracy's really only got one appropriately themed watering hole for March 17: The Shamrock, located a stone's throw from the Press. It's the one with the upside-down sign hanging over 11th Street (and yes, it's supposed to be that way).
If you're looking for Guinness on tap, however, your best bet is just a quick crawl from The Sham: Helm's Ale House, which has an impressive array of on-tap selections (and none of that thin foreign-owned stuff, either.)
So find a driver, put on your drinking shoes and raise a glass! Responsibly, of course. We don't want to see you in the daily Police Log.