I've read restatements of Epicurus' famous argument which attempt to prove that the fear of death is irrational (I don't know if Epicurus himself ever used a word like this. From what I've read, he only hints at it).
To me, it's absurd on the face of it to call an emotion like fear itself irrational (or rational). So what theory of emotions do we need to accept, that this move is even possible to us?
Let's assume the usual definition of rationality (epistemic + instrumental). We could argue, of course, that speaking of irrational emotions (something we commonly do) is just a manner of speech. What we really mean by it, is, that irrational beliefs about reality led to such an emotion or irrational decisions are caused by this emotion.
In this case, we can call the fear of flying irrational: it often results from irrational beliefs about the dangers of flying and often causes irrational decisions (if we assume desires/goals typical for a human being).
But in the case of the fear of death, this does not help. If Epicurus thought that irrational beliefs about reality are involved in the case of death, he certainly would see them in people who believe in an afterlife (and then the argument does not work anyway). Also, aside from special cases, decisions don't seem to be possible in the context of death. And what desires/goals on which basis we could judge the rationality of such decisions would we be talking about about anyway?
If I'm correct, it seems we have to adopt a quite idiosyncratic theory about rationality and emotions, so that Epicurus' argument can at least proceed. What could that be? And are such theories commonly held among contemporary philosophers? One would assume they are, since Epicurus' argument is still held in high regard.