If you're a straight-A student and still you worry about failing all of your classes, you're being irrational. Your fears are not based on fact and not likely to come true.
Usually you use irrational to describe ideas and thoughts that are not based on reason, although emotions and behavior can be irrational too if they don't seem reasonable. Do you jump onto a chair and scream when you see a mouse? If so, you have an irrational fear of mice, or musophobia. If someone takes a swing at you for failing to say excuse me when you burp, both their anger and their actions could be called irrational.