The opposite of irony is sincerity. In general, irony involves a discrepancy between the apparent and the underlying meaning of a situation or phrase. In other words, it involves saying one thing but meaning the opposite.
Sincerity, on the other hand, involves genuine and transparent communication. When we are being sincere, there is no hidden meaning and we say exactly what we mean.
For example, the phrase “What a beautiful day” is ironic if it’s cold and rainy, but it is sincere if it is sunny and warm.