If you have an irrational parent, the best thing you can do is try to understand where they are coming from. Many times, irrational behavior is a result of underlying anxiety or stress. If you can identify the source of your parent's stress, you may be able to help them manage it in a more constructive way.
It is also important to set boundaries with an irrational parent. Remember that you are an adult now and don't have to put up with their unreasonable demands or expectations. Explain calmly and firmly what you will and will not tolerate from them. This will help to establish a more rational and respectful relationship.
Finally, don't hesitate to seek outside support if needed. If your parent's behavior is truly out of control, it may be necessary to get professional help. This can provide both of you with the tools and resources needed to deal with the situation in a healthy way.