Yeah, if it is plaster, I am not sure what you can do if you don't have space to add another layer or do drywall instead.
I do recall seeing an ad for a product in the back of This Old House magazine several times for an easy to install an insulating layer that takes up less space than drywall, can't recall name of it. I *think* it is paintable.
The other option, since it is for sound and not temperature, is to put as many plush furnishings in the room as possible - at least it is a living room. Thick rug, draperies, soft furniture and pillows. Not sure if wall hangings fit your decorating style or not, but that is another layer of fabric to muffle sounds.
Hmmm, as I type...sounds like you are tight on space, but if you had any ideas about putting built in bookcases on that wall, maybe your carpenter could come up with some ideas to insulate, too.
I get it - I can hear my neighbor run down his steps beside my dining room

as well as his great surround sound system bass.