Great picture! And remarkable structures.

I don't deny the possibility of Irish Monks or anyone else coming across the ocean. My comment on the white robes was a light-hearted musing. It is difficult to keep fibers white over time. But maybe they'd found some kind of mild bleaching agent. Other people had white robes by then, so there must have been a way. This is just the preoccupation of someone who has spent time and thought on removing stains from garments.

At any rate, I like to think at this point that all things are possible, although for anyone the journey would have been long and perilous. Eventually the question may not be whether people were passing back and forth in prehistoric times, but why and when it stopped.

The structures at Oley Hills are cairns and not hollow, as I didn't make clear. But I take your point. There are many chambers in the area, and you're not the first person to point out the parallels with the Irish ones.