What irony is used in the story.

The fact that an intelligent man like George travels with a mentally challenged guy like Lennie is considered, by many, to be ironic. THere are many more as well:

-Lennie's last name is Smalls yet he is a big powerful guy.

-At the end of the book George is forced to kill the person he loves the most out of kindness (the other men were coming to lynch Lennie)

-George complains about Lennie but without Lennie he is just another lonely farm hand.