"It says Bridget of Kildare gave away all her moms butter and it magically reappeared. Am I expected to believe that?"
Yes, probably. Tho I wouldn't know what people hundreds of years ago really expect of you. Let's talk about Tests of Belief. Religions are often filled with them. A true believer is often distinguished by fullheartedly believing even the deep tests of belief. Did Krishna really move the mountain? Does the shrine really have the spirit attached? In the case of the shrine, belief in the spirit may enable further religious actions, some of which may be healthy. Belief in Krishna's story may boost belief in Krishna's divine power and resultant effects. But does religion NEED tests of belief?
What if a religion was based on neuroscience and incorporated dreams, subconscious communication, and the like? What if all the prescribed actions were based on logical reasons? It could resemble the practices of religions but without the tests of belief. Also, keep in mind that the concept, in your mind, of Bridgid of Kildare is now linked strongly to butter and, should neurons for butter be activated, so too will neurons for Bridgid (and vice versa). So there are more effects from divine stories than that stemming from whether the test of belief is passed (and by how much). You can also NOT believe the Bridgid story yet, through your words like here, continue spreading the story anyway.
So, in answer to your question, I would say whether you believe the story is not the important part of the story. You could. You could not. But the spiritual path is for shaping of the individual and this goal can be accomplished whether you believe the event true or not. At some point, you may have the option to give something to someone, think of Bridgid and the butter, and perhaps the triggered thought of Bridgid will have meaning. At that point, does it matter whether you believed it to be a historical event or not?