Thank you for your post to the Ireland Reaching Out message board and welcome to the site.
While the headstones may not be on the Historic Graves site (which is a wonderful site and project), they may be listed elsewhere.
I see that there is a Facebook page which appears to contain some of them:
I am copying this post to one of our local volunteers in the Skibbereen area who may be able to assist and know where any records are housed.
The very best of luck with your research.