I have large cracks in the plaster ceiling of 120 yr old house.

Hard to say without knowing if it's a painted gypsum plaster or lime coating over brown coat and lath. You better have a pro look at it. Or call U.S. Gypsum there in Illinois. They have answers as they are the manufacturer of the construction products used back then and are really great folks willing to help you.

From experience with my own 1854 house, those cracks are caused by the house settling. I filled mine with wall putty, but about 2 yrs later they were back. Best bet is to cover the ceiling. You can buy vinyl ceiling tiles that look like ths 1890s tin tiles. No idea of cost.

My experience is that when there are large cracks in plaster the plaster has separated from the lath making it unstable and risking the ceiling falling own. If that is the case the best fix is to pull it down and replace it.

Buy some Joint Compound and special Tape used for dry wall. Prepare the ceiling by removing any loose paint, etc. Using a wide putty knife, spread some Joint Compound over the crack. Put the tape on the compound over the crack, making sure it sticks to the compound. Then cover the tape with more Joint Compound using the wide putty knife. The next day, when it is thoroughly dry, lightly "feather" it in using a fine sand paper. Dust off the area and put a coat of primer over the area. When that is dry, you can paint. I did the cracks this way in my house 20 years ago, and you still cannot find the cracks. Good luck.

I think you need to consider the safety issue. Perhaps you can find someone to help you determine if the plaster is in danger of falling. If it is, you might be able to sheet over it with sheetrock and make it safe. That isn't, however, a quick fix. If the plaster is stable and just cracked you can repair just the cracks with a plaster patch. Look for videos on U Tube. Remember the cracks are there for a reason and it would be unlikely a quick fix will last 5 years.

Working with plaster is not easy, yes scrape any loose paint but then you need to gouge out a deeper rut where the cracks are so the grout fills it it. I didn't use the tape just small amount of the plaster at a time until was smooth. For myself I know I do a horrible job of smoothing out over the tape. Just take your time. But I think the best way is to get someone in to put dryboard on top