My husband collects ancient coins (mainly Roman Republic), and he also owns four celtic coins, two made of gold, two of silver. Like Eilthireach said, it's a puzzling and direct connection to the Celts when holding one of these coins in your hands. I can only strongly recommend people with spare time

to dip into numismatic...
There's also a dazzling piece of work by a guy called Lancelot Lengyel (well, maybe it's a pseudonym), originally published in 1969,
Le Secret des Celtes. (In german it's called "Das geheime Wissen der Kelten, enträtselt aus druidisch-keltischer Mythik und Symbolik".) I haven't checked the websites posted above yet, dunno if there are any overlapping contents - but Lengyel interprets for example a snake-like leg of a horse as a symbol for death and rebirth, the dots on the coins being symbols for the egg as the beginning of life, and so on. It's hard to decide whether it is all
correct, but it certainly may give some insights on the symbols on celtic coins.
And well, these coins are original documents, so I've been wondering for a long time why we don't try to learn more about the ancient Celts by studying their coins...

You won't get what you deserve - you are what you take. I don't know why you gotta be so undemanding - I want more. (The Sisters of Mercy)