There's a difference between anger and irritation.
Even if they are synonyms, (=can be synonyms in some contexts) they are not 100% equivalent.
Especially when you have both in the same sentence, it means there's a matter of degree. You can notice that when they give you a list of synonyms, there are often a difference in intensity.
As nouns the difference between anger and irritation is that
anger is a strong feeling of displeasure, hostility or antagonism towards someone or something, usually combined with an urge to harm.
while irritation is the act of irritating, or exciting, or the state of being irritated; excitement; stimulation, usually of an undue and uncomfortable kind; especially, excitement of anger or passion; provocation; annoyance; anger. (...) As a verb anger is to cause such a feeling of antagonism.
They both can mean tired. Why do you think it should be defessa?
My pocket Oxford Latin dictionary lists fessus before defessus for entry for 'tired'. Based on another book I have, fessa (mentioned in the book) and its forms are more commonly used (at least in more Classical writings) than defessa (not mentioned in the book) and its forms.