There's no problem, as long as you don't go shouting anti-Irish slogans or the like. Most of the patrons of these bars are American students anyway.

As for good Irish pubs, The Burren in Davis Sq. is one of my favourites (and it has trad sessions at the weekends), as is The Plough And Stars in Central Sq. (I'm not sure if this is still open though...I heard some rumours of it closing). The Littlest Bar at Park St. is also quite nice, though it's not even big enough to swing a cat in.

Other good bars are John Harvard's brewpub in Harvard Sq., Cambridge Brewing Company in Kendall Sq., Sunset Grill & Tap in Allston (I work here, and still love it! 112 beers on tap!), The Publick House in Brookline.

Andrew.