Plasterboarding between exposed beams

[DOLLY]

We have removed crumbling horsehair/lathes from 2 of our bedroom walls in our timber framed cottage and would now like to fix plasterboard between the beams so that we have the beauty of the exposed beams rather than just plasterboard over the top. The question is how do we do this? How do we fix the plasterboard between the beams? do we insulate between our bedroom wall and the bedroom next door? There is a bedroom either side of these two walls which at the moment we are looking at the back of those plasterboard walls.
Really appreciate your help and advice.

[jake-the-peg]

I would fix some wooden battons to the side of beams, just a bit inset and fix the plasterboard to those.

You can put insulation in before the plasterboard if you want to.

[Clanad]

Kinda depends on how far apart the beams are. Standard framed walls require each 2X4 stud to be 16" on center to give the drywall (as it's called here in the U.S.) some sub-structure strength. You could do as jtp suggests even if you frame up the 2X4's as long as the exposed beams are wider by at least 2 inches than the 2X4's...

[The Builder]

I'm just guessing, but I the type of wall I think you have is not normally seen in the US, Clanad :o)
Lath & plaster over funky old studs ........... if it's really old, they might even still have the bark on them!!!
Jake has suggested what I have done many times. You'll probably have wall thickness available to take 50mm battens only. (That's 2 inches for Clanad Lol)
Fill the 50mm space with "Rockwool batts" .................. it's insulation, but it has better acoustic dampening properties than other fibreglass batts.

[AlBags]

I work on this type of thing continuously.
As said above really .. the timbers will be pretty hard, so nail support battens in with cut nails if poss. Seal/stain timbers after blasting/stripping before wallboarding.
Make templates with card if necessary so you get a great fit .. the old timbers will not be parallel or square.
Gap fill with a gun and ACRYLIC (paintable) sealant. Don't get silicone anywhere near any of it! You don't need to plaster if you have no joints .. use 1/2" wallboard if possible to help with noise.