Perhaps "irrational" isn't the best word for describing Littlefoot's state. Even your post contradicts itself, DH. You say you feel he wasn't irrational, then you describe his emotional state, and then you describe such behaviour as irrational in your last paragraph.
In fact, what Littlefoot was doing was "rationalising".
"To rationalise" is "to invent plausible explanations for acts, opinions, etc., that are actually based on other causes". He tried to come up with an explanation for the events that unfolded, but he doesn't arrive to conclusions a person which is not under stress would arrive to (for example, that the sharptooth would have come near the herds regardless of what he had done, or that nobody, including his mother, could have known this sharptooth to have been so unusually strong). Instead, he blames his mother, then himself.
Now, is rationalising an irrational behaviour? Rationalising uses the tools of reason, as we can see with the question "Why did I have to wander so far away from home?", but it is guided by the subconscius, which is irrational (lacking reason) by definition.
Then again, rationalising can be done on purpose, at full consciousness, to justify one's own actions. This definitely does not apply to Littlefoot here.
So, Littlefoot was being irrational, in the sense that he wasn't fully aware of what he was saying or doing. It is normal to act in such a way in such a situation.
