I must admit that I too wondered about this building a few years ago. I never learned anything definite but there were a few hints that it's probably a private cottage and therefore likely to be locked. I could be quite wrong but if anyone has any superior knowledge, I'd suggest that they contact you off the forum. If it is an open bothy, it wouldn't be wise to share the information on a publicly accessible site.

Regarding the coastline walk, I'd imagine that the way north to Glensanda will be extremely rough so a higher level or inland route would probably be preferable. There are paths shown on the map north of the superquarry but I can't help on how walkable they may be. There's also the question of passing through the working quarry and I'd be inclined to check with the company what their attitude to public access is like. I'd guess that it may well be fine but it'd be a shame to get there and find that you're denied passage.

Colin