[weegee]

What is the word (not one that you made up) for one who believes in leprechauns?

God is a universal concept shared by remotely separated people. They may not all share the same concept of God, but know what each other is talking about when you say "God".

Leprechauns are regional folklore from Ireland not shared by other societies.

[garbanzo]

What is the word (not one that you made up) for one who believes in leprechauns?

A leprechanist. Theist is just as made up a word, it just wasn't made up by me. In fact, the word "atheist" was first used by the Romans to describe Christians and others who did not recognize Casear as god.