I've done a lot of hunting for the correct name, but nothing official seems to jump out of the woodwork.

The only definite seems to be the term for the occupant, who would have been a fire watcher (two words, no hyphenation), which probably reflects the age of the term, as I had been guided by firefighter, and fire-eater. Presumably their more common use led to their development.

http://www.for.gov.bc.ca/tasb/legsregs/archive/fpc/fpcaregs/ffirepre/ffpasr.htm#4

Now, is it a hut, or a post, or a lookout, or a... ?

I favour "post", as in "The position where someone (as a guard or sentry) stands or is assigned to stand", so the fire watcher can be provided with a hut at the post.

Can we approximate a marker position for the remains?

And lastly, a location name. Bishop's Glen seems better than Dunoon, even for the rifle ranges. Would this be better for the ranges, and applicable to this post?

Fire Watchers Post seems OK

It is shown on the OS getamap at NS 161763  Our Latlon Map is very fuzzy at that point and it does not show up.

Bishop's Glen is OK for the titles as long as Dunoon comes into the blurb.

Apparently there's a run on apostrophes at the shops today - Bishop's Glen Fire Watchers Post  

getamap's as rough as badger... you're looking at a possible error of around 200 metres, and that's on top of any error from not being able to identify the point of interest.

You can use our old pal RCAHMS and the Pastmap view to get near spot-on fixes for locations.

If you pan and zoom the map until you're close enough in to eyeball the point of interest in, then all you have to do is put the mouse pointer on the spot, then look down at the status bar of your browser - along the bottom of the screen - and you will see the grid reference of the point read out there continuously.

This is very useful for features that are not identified, or don't appear in the aerial view, but you know exactly where they are on the ground.

...the area of Bishop's Glen, west of Dunoon...

The peg for the Firewatchers hut is a bit out.  It should be here http://www.multimap.com/maps/?map=55.9444498,-4.9463869|18|8#map=55.94513,-4.9446|18|8&loc=GB:-0.1261:51.509:8||United%20Kingdom  assuming that this will take you to the same point.

Sorted (told you the getamap view was rubbish for that job ).

If you're quoting a Multimap link, because of the character combinations they use, it's best to wrap it inside url tags (9th from the left in the toolbar above):

[ur1=http://www.multimap.com/maps/?map=55.9444498,-4.9463869|18|8#map=55.94513,-4.9446|18|8&loc=GB:-0.1261:51.509:8||United%20Kingdom]hut location[/ur1]

Note that I've used ur1 rather than url above, to prevent the example being changed into a link.

Which shows as hut location in the message.

It's actually important for Multimap links, because the use of the | (pipe) to format their links means the open url is inactive after that is read, and if anyone clicks on the link, they get taken to the point at which you first opened the Multimap view, and not the one you eventually panned to and wanted to provide the link for.

I hope that made some sort of sense