Sonny Mclean's is probably the most famous Boston bar here in LA, but it's pretty small and a very tough place to get into. They also play a bit of favoritism with the older regulars and seating. If you're going, get there really early and stay a long day. Their menu is pretty good -- they even have whole fried clams (with bellies), which is very rare for LA.
Fat Face Fenners is okay, though getting a parking place in Hermosa on a summer weekend is rough.
If you want to go over the hill, Timmy Nolan's is pretty good, but also a tough place to get into. Watched game 1 there and it was packed with Boston fans. It's on the Burbank/Toluca Lake line (on Riverside) and has good pub food and lots of TV's.