Hi Barb: Although I've repaired many, I'm not an expert on old plaster, except I have never seen old hand-set plaster walls that are smooth....it is what really adds to the character of an old house. I guess the question is..how lumpy? If you put a 4 foot level up against a wall with the middle of the level on the biggest 'lump' and each end of the level now sticking out an equal amount from the wall, how far would those edges be out 1/4"? 1/2"? I've tested this on several of my hand set plaster walls and found the walls not to be out more than 1/8" in any 4 foot run....and these spots when side-lit, are definitely wavy.
Yes, the plasterers of old applied all the plaster by hand and finished it by eye, and most were very good at it. However, being human, I'm sure they, on occasion, had bad (or maybe great) previous evenings that caused disorientation, dizziness and nausea the next day, leading to some really lumpy walls :-)
Seriously though......unless the lumps are really obnoxious, I'd leave them alone. The shinier and brighter the paint finish, the more evident the imperfections of a wall will be. If the lumps are more than you can live with, have a plasterer (not a drywaller) come out and take a look. He should be able to chisel out the protruding plaster and patch back using the same kind of plaster in such that you can't tell it was repaired.