I had a square opening between the living room and the dining room that we wanted to arch. Since the walls were 1/2" drywall, I cut out 1/2" plywood. Then, I 'braced' the arches on each side with pieces of 2x4s. Then I 'slit' the back side of some drywall to fit the arch, thus, allowing a 'flat material' to bend around the arch. Then I used plastic cornerbead, then I mudded and sanded quite a few times to get it just right. Of course, I finally painted!
I used screws on everything; no nails. It was pretty easy given that there's already a 2x4 across the top of the arch and down the sides. IF your current 'opening' doesn't have a 'short wall' on one side, you'd want to consider building one. The alternative is to 'cut' into the wall and, well, put up 2 2x4s for that one side of the arch. It's a mess but if room is an issue, that's what I'd do.