well hard to say what falcon symbolizes to celts in that time
but i know my country had some old songs (unknown how old it is) in with falcon is included as the main bird
my country is baltic actually but 2-3 century BC here is no info who might be living here
falcon as i know was a quite popular in the medieval times
but more or less i think it just one of the posibile animals who was used on the celtic helmets at that time
we have checked all our information and references we had and the real thing seems to be that at 3-2 century BC celts more used metalic constructions on the helmets (like animals, birds, horns)
and it is very less information about celts using horse hairs (in 2-4 century BC) or a crest on helmet top like greeks did it
mainly here is the main references i used of that helmet to make
the helmet will be used only by heavy cavalry
and some variation with falcon going to be included in a (probably) new celtic noble class with is going to be something similar like romans had centurion class