"Is there a tendency for bullauns to occur more in soluble rocks like limestone? Or radon rich granite?!"

Bloody good question! I've seen sandstone and grantite ones. Proportions? Dunno. Churchy ones have a habit of being sandstone (I think). These are more usually in boulders and single.

Most multiple ones I've been to are in bed rock and are granite.

There is a certain amount of folklore from both Scotland and Ireland concerning using depresions in both cups and on standing stones to pour milk and other offerings into to ensure fertility and good milk yields and appease the local spirits such as the Gruagach.
I would recommend having a read of Paul Bennetts article Called "Cup & Ring Art - It's Folklore, Myths and the Shamanic Persepective".
I'll see if I can find an on-line copy and post a link.
If not I can send it to you
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