"Who drinks milk?"
Translation:Cé a ólann bainne?
For the same reason that it's not cé bainne or cé madra - cé bhfuil just doesn't make any sense in Irish.
You will encounter the phrase cé go bhfuil, but it has nothing to do with "who?" - in fact, it isn't even used to ask a question, it can usually be translated as "even though".
Bhfuil is not a particle. Cé invariably needs a particle in most cases at some point in the sentence. (Why? would require a long dissertation. Look up relative clauses). Duo's sentences are somewhat simple so the particle appears early, but it is there at some point in most sentences. "Cé mhéad uair a bhfuil an Corn Domhanda buaite ag an mBrasaíl?" This is a very complex sentence that use the particle down stream a bit. It means, "How many times has Brazil won the World Cup".