What is ironic about the meaning of his name?

Judges 3:8 (ESV)

8 Therefore the anger of the LORD was kindled against Israel, and he sold them into the hand of Cushan-rishathaim king of Mesopotamia. And the people of Israel served Cushan-rishathaim eight years.

For Israel’s wickedness (Judges 3:7), God sells them into the hand of someone who is doubly wicked (which is what rishathaim means).