Thanks for you post.
The easy and fast way is to Demo the plaster and install wall board. The major question is how wide spread is the problem. If it is just in the area of the corner, the plaster may be worth saving if it is a small part of a room which is in otherwise good condition or has...
The answer to you question depends on the length of span, size of joists and if there is any load bearing partitions with the range of the span. Need more info.