How to insulate attic areas that are unreachable
If you determine that additional insulation is needed, you have a couple of options:


Cut into the ceiling for temporary access, install your insulation, then patch it back up.
Use a product that doesn't require full access to install, such as blown insulation or some spray foams.


Blown cellulose is most likely your best bet here as it is low-cost, fairly effective, and can probably be blown across the top of your dormer from the access hole you already have in the attic. When you buy the insulation you can also rent a blower machine that takes the insulation and blows it through a long hose, kind of like a vacuum cleaner in reverse. This allows you to inject it into places where you might not have full access.

If needed, you can also blow it in from the room below by drilling small holes in the ceiling. These would be easier to patch than if you had to take the ceiling down completely to install batts.