The reason why Merlyn has always been portrayed as a mystic druid-type character is mostly from the writings of Geoffrey of Monmouth. I am under the impression that Druids in Britain were far different from those Caesar portrayed.

First of all, the OI word for sorceror is very close to Druid, but does not imply that they were the lawmakers and healers that many think they were. The Irish had their own Brehons who made laws, as well as people designated to be bards, healers, and druids. From the stuff I have read in Irish folklore, druids were not as well respected as many of us have thought.

The connotation between Merlin and druidism is a problematic one, just as druidism in Britain is. Tacitus describes them as being more for the purpose of scare tactics more than a powerhouse of wisdom.

It is likely that druids in Britain were merely shamans, and most likely marginalized once the Romans came, which would explain why the tale of Myddrin tells of a madman, and not a clever, analytical thinker many of use connect him to.