So, you're in Harvard Square Cambridge. I don't visit there as much as I did when I was young, because it's become almost impossible to park there, so I don't know that many places in the square itself.
While I think the Cambridge Brewing Company is the Brewpub to go to around there, and it's only about 10-15' away by subway... You're within a stone's throw of the John Harvard Brewpub. I was there last month and they had a really nice Bitter on tap.
You're just down the street from a Redline T-stop, so any of the places I mentioned near a Redline stop are within about a 5 minute ride. They're actually close enough to walk to, with a little ambition.
There's another good beer bar really close, maybe a 10 minute walk north on Mass Ave. It's called the Cambridge Common
. It's on the right side. It has somewhat typical pub fare and a good rotating beer list. Pay special attention to beers from Pretty things and Berkshire Brewing, in my opinion, but there's usually a lot of good beers there, like Allagash. My wife and I go to the music club downstairs, The Lizard Lounge, a lot to see bands and it has the same beer list.
The other way on Mass Ave, perhaps a bit further but around a 5' walk, there's a famous Irish bar called Plough and Stars. It's often said that Bonnie Raitt got her start there. It's changed hands a couple times in the last couple decades, and it's not as much of a dive as it used to be. Nothing much for a beer list, but a lot of people would claim it's the best place west of Ireland to buy a Guiness, which is pretty much the national drink of Cambridge.
I don't think I mentioned anything in Central Square, one stop south of Harvard Square on the Redline, or about a 5' walk past the Plough. There's a couple fantastic dive bars there and a few good places to go for a $8 Indian buffet at lunchtime. There's a really atmospheric Irish Pub called The Field right in the center of Central Square, right where Mass Ave crosses Prospect Street. It's just behind the Starbucks, on Prospect Street. That's where I go when I want a Murphy's.
If you continue down Mass Ave another few minutes, maybe about 3 blocks, is a big pub on the right called Asgard. It's in the first floor of what's probably an office building. It's vaguely Irish in theme, but it's atmosphere is pretty fake. It's got a good list of beers, though.