In the future, everyone will have a detailed psychological simulation model of all their interaction partners, which can predict their responses with high accuracy. Negotiations will be simulated and tuned by bouncing models off each other, later the simulation becomes recursive.
If the irrational behavior is deterministic, it can be modeled like rational behavior in that it constrains the behavioral options of an agent. If it is indeterministic, it limits the ability of the agent to optimize. You may have to consider more of the agent's bad options then.