This site is well worth the extreme difficulty of reaching it (three ferrys and an open boat with an outboard motor mimimum). Ferry 1: to mainland Orkney, Ferry 2: Kirkwall - Westray, Ferry 3: Westray - Papa Westray, Small Boat: Papa Westray (Papay) - Holm of Papay. Enquire at the Youth Hostel/Co-op for boat trip (a few pounds).
Site is very visable from Papa Westray - looks like a submarine due to upstanding concrete entrance through roof. Roof has been restored with concrete and glass bricks. Muddy floor is covered in plastic sheets, which is useful but ugly.
Worth visiting for the sheer number of antechambers (14 or so from memory). "Eyebrow carvings" are less impressive in reality than in photos.
There is also another, round chambered cairn at the north of the Holm, which is less well preserved (ground plan only) but easily found, and worth visiting.
Incidently, there is a natural rock bridge in the cliffs on the south end of the holm.