This is not what it sounds like.
Starting in the early 1700's in various educated circles, it ran out of control, peaking around 1850's. It stayed mainly in The British Isles, but was seen in some areas of The U.S.A around the 1840's to 1860's.
This pseudo-renaissance was an attempt to bring back Druidism and resulted in the corruption of historic sources to fit into a romanticized concept about what Druidism should be.
The OBOD, The Order of Bards, Ovates & Druids, owes much of its roots to this blatant disregard for the Truth. That is why they do such out-of-ideology things as offer mail order "How to Be a Druid" kits.
I am not very fond of this. This a near-rant write-up, and I apologize.