I think fear of dying as in suffering the body dieing is irrational. Fear of death itself and what might come after is not irrational because fear is there to protect you, doesn't it? No one really knows what's after supposing something actually happens when using the word after. So really, all we know is that we are living right now and that it will stop because we have been aware of others dieing.
Let's take a more creative path of discussing death, and focus on the facts and the theories. The facts I already stated, so we already know we are just assuming and discussing something can happen after death because that's how we come to talk about it. Im most fond of buddhism. Some idea is that we are all god playing hide and seek with himself(notice how when we usualy say him its like we are talking about a guy) for the fun and drama. In my opinion it makes the fear of death rational, because you aren't suppoused to know what will happen anyway to keep the game interesting.
My other thought is, and think about it really... What states of conciousness do we know? We know you can experience being alive and you can experince differently using cacti and chemicals, NDE induced by the DMT, and the closest to death is sleeping without dreaming. If you don't take time to wake up it's almost like you took a few seconds to sleep. If you take your time, you wake up still a little dosed with dopamine and whatever else in your head that make you feel you're in place and comfortable so you feel like you've been there long. What we know is even when we didn't experience anything, other people did and we didn't just because we were sleeping. I think this implies that the origin of existence can't be traced with perfect measurement, because all that you can know is what requires YOU, a person, an ego. If you are not a human being, you can't experience yourself because a self doesn't exist and neither do you. If you can't define yourself then you're just happening, not defining, not aware this way that we are. The way this existence is happening, saying you'll be in heaven or hell after you die is basicaly the same thing: after you die, you will still be alive elsewhere in endless fun or endless lameness but after you grow up you know there isn't endless fun because everything turns boring after a while. Also, do you really think you'd like to sit down at a pretty garden for 1,000,000,000,000 years for starters with your family and adam and eve and bring up everything that happened so fucking long ago? It wouldn't matter anymore... It would get annoying probably and people would make trouble, so it turns into hell eventualy.
But think about it, after discussing all this, we already know you can know if there is you. You can only define death/no experience/dreamless sleep before and after it occurs obviously. So... If we had a soul eventhough its hard to grasp, but no YOU, will your soul just experience angels or demons? This makes existence not worth existing. So if you're still with me, how about this: I have an idea what happens after my death and your death exactly like I have an idea what happens when I'm asleep, because there is always the "I"(No, no soul, just a human being defining) when things are happening. We know that when we were asleep it didn't count for what's happening in the real world if we experienced nothing. But we can say stuff still happen because we know other people were around, up, and defining away exactly like us.
So... What happened that counts? After the first president of america or the 100th human being to exist? We can say we did because we are experiencing this now, and we know it. Will it help us to survive to know every event between them and us? No. It's basicaly like saying what happened in the real world the second you were born the second an old guy died, or a second before. Really we know... Its us. So, reincarnation? You could define and call it that. Why? Well, remember the reason you simply can't experience being someone else or remembering you were someone else is because you are already here now, and this is the way we work. Only after years of living we can calm things down with our defining, seems pretty even to me.
This opens you up to the idea that we are just running down deep imaginary rabbit holes being afraid of death. But if you are afraid of death you already know its possible to not fear it.
fear of death is not irational.... image you spend two hours making delicious food and you need to poop so you go to bathroom but in middle of bombardment you remember you left the whole batch of food in easily accesible place where your dog can eat it?
you would be scared to loose that food,why would you not be scared to loose yourself?
If I wasn't scared and got back without focusing much on the fact the food is gone and just acted like nothing happened and started making food again or did something else, you'd think something might be off about me. Right? That's not rationalism. Rational would be fearing or feeling down about the food because you know you'll to decide wheter to go through making it again repeating the same work because of your mistake. The fear reveals to you the next courses of actions you might have to take and motivates you to do something about it. I think in the same way fear of death motivates you to get life right this time because you feel you're gonna have to start over again sooner or later.