Geoffrey of Monmouth wrote of Merlin building Stonehenge and transporting the stones from a circle at Killaraus in Ireland named the 'Giant's Dance' in his 12th C book, 'History of the Kings of Britain'. Which I think is one of (or maybe the) eariest written reference to Stonehenge.

You can find passages on-line here...

http://www.lib.rochester.edu/camelot/geofhkb.htm

BOOK VIII.

CHAP. X.--Aurelius is advised by Merlin to remove the Giant's Dance from the mountain Killaraus.

CHAP. XI.--Uther Pendragon is appointed with Merlin to bring over the Giant's Dance.

CHAP. XII.--Gillomanius being routed by Uther, the Britons bring over the Giant's dance into Britain.

Andy