Barry Fell noted that this particular image from the Gundestrup Cauldron is holding up his hands and spelling “thunder” in Ogham — and the figure is a thunder deity. One view (which IMHO is correct) is that the method of writing began as fingerspelling, a way to communicate silently and (if others were unfamiliar with it) secretly.
http://www3.bell.net/sacredwheel/gundestrup.jpg
Gaelic in Eire was not taught to outsiders,in the 18 and 19 century the brits outlawed the speaking of the gaelic language..
So it went underground,when i went to school they taught it as a second language.
I believe it is dead now,we are all Europeans...