Is it prehistoric? And what period of pre-historyes it come from? Has it been excavated?

I didn't know about this one until know ... it's lovely. I wish they'd excavate a few of the 'clochans' around Cork and Kerry. There they have intact beehive huts that are assumed to be early Christian (used as over-night camp sites by pilgrims on the way to the island monasteries). Looking at some of them I think they could be older.

We know that corbelled roofing was used in the Neolithic, but the builders usually relied on the cantilevering weight of the cairn or mound covering the chamber to hold it all together. A freestanding corbelled roof is quite a technological achievement.