I don't think so. Any projectile of decent mass will be deadly when accelerated to high speed. There's nothing that magically exempts potatoes from this just because they're edible. You can absolutely kill someone with a potato gun. The prosecution in question may be wrong for other reasons (e.g. fired safely and not with any intent to harm), but "potatoes can't kill, because otherwise Ireland would be a superpower" is not the way to argue it.