What is your question? Are you contemplating DIY, or are you asking if it can be fixed? If you want to know if it can be fixed you should look for a contractor who can do the work to give you a quote. It is hard to find these people as this is a dying art. If you're contemplating DIY, time to hit YouTube. We used to live in a 100 year old house. My husband, who is a pretty expert DYIer, says the second picture you showed is probably fixable, but the first picture will present more difficulty.

Your other options are to tear it out and replace it with drywall, or putting drywall over it is sometimes done. We were old house lovers until we lived in one and tried to keep it in good repair. It was too much work. While you may be interested in being period correct, unless you have a landmark home you might be better off doing some shortcuts before you go broke or get very frustrated.

Were it my home I would rip the walls off and start anew. It is only a matter of time before it all comes crumbling down.

Any sign of water damage? If so, check for mold.

I didid ssome research on plasplaster and olold homes. The plaster is a much safer option in fire prevention and damage

and i’ve lived here for 36 years and this is the first time that i’ve had to do this type of repair. the reseach said that typically in old style plaster it starts to losen about 100 years in unless there’s water damage which would cause damage earlier i’m going to try to do as much as I can mysel

Try to find some old house forums and/or local people who do plaster repair. Beware that people whose skill set is drywall are going to tell you that it all needs to come out. Very rarely does all plaster have to come out unless its been neglected or water damaged over a period of time. It may be you use a range of different techniques depending on what kind of damage. You may find that some walls have only minor cracks that can be simpy filled ,while some walls may have major bowing and large loose areas. arger damaged areas can either be re-done using old methods OR.... just attach drywall patches and fill in gaps with plaster (using bonding agent to get old plaster to adhere to new plaster).

Whatever you do, please dont do the thing of putting drywall OVER the old plaster and butting it up against the baseboard and window and door trim. Thats just plain ugly

Ceilings might be the one surface you decide to have drywalled - looks like we are seeing ceilings in your pics? Gravity has greater effect on ceilings, so damage can be a lot worse than walls. And, since theres no trim to contend with up there, its a lot easier. If you want to start a thread and post some pics over on the old house forum, I think youd find a lot of us could advise in more detail.

Are you in the USA?

Would remove all the dry wall, check for problems that might need to be repaired, add insulation to exterior walls where needed and use light colored 4'x8' paneling for the walls.

https://www.lowes.com/pd/Oak-47-75-in-x-7-98-ft-Embossed-White-Oak-Wall-Panel/1000937538?cm_mmc=shp--c--prd--mlw--ggl--LIA_MLW_135_Windows-Patio-Door-Garage--1000937538--local--0-_-0&ds_rl=1286981&gclid=Cj0KCQiAzMGNBhCyARIsANpUkzOCE06tbY8tXC9-PwCc32xmKkjDzUYdjkNcEsKkbVp_tl6MmSTK3wcaAkO9EALw_wcB&gclsrc=aw.ds

When was the wiring and plumbing last inspected or replaced? Can most of the wiring be seen beneath the floor and/or in the attic?

Have you had any problems with any electrical outlets or lighting? If so, after you remove the drywall in the room(s), would be a good time to check any problems and, for safety sake, make any needed repairs before hanging the replacement drywall ... whether sheet rock or paneling.

If there are no small children in the home, you might consider removing all of it and replacing it with paneling one room at a time.

On a budget? If you're considering whether to use sheet rock or paneling, take into consideration the cost of having sheet rock finished and the time involved as you compare that cost with the cost of paneling.

If most or the walls have cracks, it could be that the house itself is settling and covering up the problem with more sheet rock will certainly not solve the problem -- only waste money as the problem reoccurs. Can you get into the crawl space beneath the house to inspect it -- or have that done?

Suez, OP's 100 yr walls are original plaster and lath, not drywall and he/she is askig about repairing it.

Yes, "plaster is a much safer option in fire prevention and damage," per inspectors, it prevented my house from burning to the ground during house fire.

Also great for sound and heat/cold insulation!

Another plus for plaster is the surface is harder and doesn't get dings and damage nearly as easily as drywall. Removing cracked plaster is not an upgrade.

We have had good luck with drilling holes through the plaster, squirting construction adhesive between the lathe and the back of the plaster and then screwing the plaster pieces to the lathe. After the adhesive dries, you can remove the screws and then patch the holes and cracks. If you choose to drywall any ceilings,  use lots of adhesive and longer screws and go right over the plaster if the plaster is not crumbling.