The fact that you worked at all may help a bit. It also might distance you from a sub-par GPA. Overall, the fact that it was a law firm won't be particularly important above and beyond the fact that it was a part-time job to begin with. In fact, I would even avoid talking about it much personally, as law schools tend to look for something "special" that sets you apart from the crowd. That's just me, though. I'm not on an admissions committee myself so take my advice w/ a grain of salt.