Be Careful!!
If you plaster over it, the additional weight may cause the plaster under it to come away from the wall if it is defective, on a ceiling i have seen it collapse under the additional weight. For a long term solution, if you are staying at the house, get it off and start again, if the plaster comes away as well, patch it up. Plastering is quite cheap to do if you do all the prep work yourself, although a bit messy, it is really only a weekends work, and use plenty of dust sheets. REmember, the job will only be as good as the effort you put in, that is how it ended up in such a mess in the first place, take your time...and do it right