Thanks for the photos. That is definitely man-made. Unfortunately I have no idea why these structures are there. Now I have to add another walk to my to-do list so I can have a look at this for myself.
It has been several years since my last trip up Myuna Ck so my memory is a bit fuzzy but I am pretty sure that is the spot.
According to Neil Patton (in his book Sydney and Blue Mountains Bushwalks, page 136) there are remains of a building in that area. I think you did find the ruins mentioned by Patton. Well done. I did look for them once but I am confident that I got the location wrong and I wasn't in the right spot.
The scrub has gotten very thick and I am sure the weak footpad on the true right of Myuna Ck would be completely overgrown. There was a fire trail that started on Woronora Dam Rd and finished on top of the ridge to the south east(ish) of the intersection. Could be an alternative way to the Myuna/Ripple Rill intersection. EDIT: Just had a look at Streetview along the relative bit of Woronora Dam Rd and the scrub looks very thick. Might not be a good way to go.
Sorry I couldn't help with the Tarmaroo Rill mystery. Hope that my info on Myuna Ck may be useful for future adventures in HNP.
Cheers,
Michael.
One foot in front of the other.