I was there in July 2016, and organised a sunset visit after the park was closed, allowing access to the inside of the stone circle. If you have time to organise it, do it! I used Mad Max tours.
DFA15-30 @ 22mm on the K-1
@ 15mm
DA10-17 @10mm on the K-1 (crop mode)
From further back, but still inside the rope. DA*55 on the K-1 (FF mode)
So essentially, if you're looking for primes on a crop camera, 35mm (maybe even 40mm) will get the whole stone ring in the frame from the security rope. Something a bit wider would be perfect if you want a bit of wriggle room. I reckon he DA21 would be perfect, especially with its flare resistance. The DA15 would be too wide.
Personally, despite the fact that light will be scarce at that time of year, I'd pack a zoom (18-55, 16-85, 18-135 - whatever you have) because I imagine there will be quite a bit of competition for positions. We had the privilege of avoiding that - there were only 16 of us in the whole area