Irrational just means to act on instinct (model-free reinforcement learning) as opposed to planning (model based RL). Planning is more deliberate, can incur model errors and takes more energy, but instinct lacks generalization power. Both are useful systems of behavior, the problem is when one is triggered instead of the other - specifically, planning is useful in novel situations, where memorized responses don't work. Instinct is better where situations are repetitive and when uncertainty about the situation is too high to use modeling.
