Is disregarding normative claims (epistemic like "you ought to believe true things" or moral like "you ought to donate to Oxfam") with full knowledge and understanding of them irrational? Is rationality grounded in other way than that we just all seem to participate in "rationality project" from the get-go (question of "why be rational" seems to be self-defeating as we look for reasons to be rational..)?

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For the first question, let's

For the first question, let's focus on the practical claim. Suppose you judge that, all things considered, you ought to donate to Oxfam today but you experience no motivation to do so. You never form the intention to donate or form a desire to do so. A version of what is called "motivational judgement internalism" holds that this psychology would constitute a rational failing. This version maintains that it is a rational requirement that whenever someone judges that they ought to A, they experience at least some motivation to A. I won't take a stand on whether this thesis is true, but this should give you some terminology to search for.

Your second question asks what explains rational requirements and why we should follow them. One strategy is to try to show that all rationality just is instrumental rationality. Since it seems unproblematic why (all else equal) we should take the necessary means to our ends, showing that all rational requirements amount to this would be progress.

How would this strategy work with epistemic rationality? Consider the putative rational requirement not to have inconsistent beliefs. Following this requirement is a necessary means to satisfying our cognitive goal of not believing false things. For, having inconsistent beliefs guarantees that one believes something false. So, according to this strategy, there would be an instrumental reason to be epistemically rational.

Now, I'm not saying that we can successfully reduce all of rationality, or even epistemic rationality, to instrumental rationality. This strategy may not work in the end (see Thomas Kelly's paper "Epistemic Rationality as Instrumental Rationality: A Critique"). I merely wanted to give a brief sketch of why someone might give an affirmative answer to your second question.

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