The guys who did Fourknocks did use a concrete bowl that looks really ugly when lit up but in the gloom its not noticeable, the small shafts angled to light up the best carvings are pure genius though! http://www.themodernantiquarian.com/post/40927/images/fourknocks.html

It's a much better job than at Knockmany http://www.themodernantiquarian.com/post/44543/images/knockmany.html

'Through a chink too wide comes in no wonder' as Patrick Kavanagh would have said if he had visited Knockmany.

Keep the skylights small so you can close the door and let your eyes adjust and let your imagination get into a comfortable position, that's what I say!

Some of the unique carvings at Loughcrew are probably seeing their last century out so though it would be totally unfitting for something clinical and 'public toilet' like up in the wild hills, maybe temporary wooden domes weighed down from the top so they don't need foundations could be an answer, covered in a layer of soil so grass will grow.