My house was built in the late 1800s. The curve under the stairs?

What is your question? do you want to disguise the crack in plaster?

I would love to just cover up the plaster at this point. I need ideas on the curved ceiling and all the plastered walls going up the stairs.

I had an older house once. I completely understand your struggle—- gotta love that lathe and plaster. If I could just get some of that money back. I wonder if it would be easier to make a box shape that will fit into the area and then feather out the drywall to the wall in every direction. I know what it’s like to get rid of some of the features that you enjoy but in time if you can’t figure out how to use it, it may be time to lose it. Good luck

I think I would plaster the cracks, and then to smooth out the walls if needed skim coat them with plaster. Another option would be to use a roller on a long handle and use Kilz High Build.... https://www.amazon.com/KILZ-Surface-Interior-Water-Based-1-gallon/dp/B000LNTLX8

repair cracks... https://www.youtube.com/watch?v=9e4RVfbNBsA

https://www.youtube.com/watch?v=1FqlnWdexIU

https://www.youtube.com/watch?v=y9pkIe4ClxM

As to making storage. I would scribe a piece of birchfront plywood, and cut the plywood to the angle of the curved ceiling to make a cabinet face, then cut out a rectangle or square to build a cabinet with big storage drawers. https://www.youtube.com/watch?v=S3mtv-KEbEs

If at all possible, I'd want to keep the look. It's a feature that is so rare, it's beautiful. I'd highlight it instead of hiding it. Remove the items you have there, finish the wall to match (or highlight it as well), and make it a nook. It would really look spectacular!

We owned an 1839 Greek revival with similar plaster issues. We used plaster "buttons" to shore up the cracks, applied a few coats of joint compound, and repainted. Check out this YouTube. You might not have to do every step it shows, but it will give you some ideas that will work on the curved part instead of trying to cover it up.

I think the curve is lovely, and wouldn't want to change it. Plaster, on the other hand.... I grew up with plaster lathe walls and know the beauty and beast of them! Address the cracks first before they really start to crumble. I, personally, would consider wallpaper for the ceiling of the curve and like wainscoting up the stairs. That would also help hold the plaster, and wallpaper today is really gorgeous... Much better than in the past. Google "storage under curved staircase" and there are some great images for ideas. 😉

If I understand you correctly the cracks are a side thought your real issue is you want storage but the curves make it hard and you want to disguise that look. I would build a “box” shape that would fit right up against the walls and the ceiling that way you have a square space. Just like if you have a circle and you want it to be square you put 4 straight edges along the curves so the middle of the flat wood touches the wall that sticks out the farthest even use firring strips the top may take 3” and the middle only 1” withoutpics and measurements I couldn’t tell you more

If I understand you correctly the cracks are a side thought your real issue is you want storage but the curves make it hard and you want to disguise that look. I would build a “box” shape that would fit right up against the walls and the ceiling that way you have a square space. Just like if you have a circle and you want it to be square you put 4 straight edges along the curves so the middle of the flat wood touches the wall that sticks out the farthest even use firring strips the top may take 3” and the middle only 1” withoutpics and measurements I couldn’t tell you more

Are you talking about the small space directly behind the steps? I've seen "hiding spaces" created by making drawers that slide out from the face of the staircase. https://evduzenleme.com/esyalari-yerlestirmek-icin-muhtesem-fikirler/esyalari-yerlestirmek-icin-muhtesem-fikirler-11/

Or you might be able to make shelves behind the steps with trays that pull out (or drawers) so you can make use of the depth. Then the area beside the stairs could be used as storage in the same way depending on how much room you have. Here's another version. https://www.pinterest.com/pin/102949541462223492/