My usual language is french
However, looking at very old time, ancient civilizations was practicing good adorations, unless greek adoring man's representations of goods, celtic were adoring nature, like the representation of flowers, limbs, stones and so (this was the major guides for artistic feelings of Celtic. Other way the natural environnement was invitating or disturbing, one of the invitation was water fountain, because of the clearness of fountain, the healths benefit of clear and non polluted water. So Celtic people were used to live close too a fountain and people name was fountain. More later after the Roman greek period, roman accept the JC religions or catholic if you prefer. Catholic preach tried to capture the benefit of fountain by binding a Saint name to the fountain to make it a sacred place and/or pélérinage place.
This is the origine explanation of the name that large population carried.