In Browning's "My Last Duchess," the speaker says, "E'en then would be some stooping; and I chose never to stoop." Is this irony or not?

The Duke is telling the envoy about the fraught relationship he had with his late wife, the Duchess. In the course of the discussion, the Duke reveals aspects of his wife's personality and behavior which he found particularly annoying. When his wife was alive, the Duke never saw fit to inform her of the seemingly inexhaustible ways she displeased him. Yet he has no compunction whatsoever in revealing such information to a complete stranger:

E’en then would be some stooping; and I choose Never to stoop.

In other words, no matter how disgusted the Duke was with his wife—highly disgusted, as it turns out—he never stooped so low as to discuss the matter with her. The Duke clearly thinks this would've been highly improper for someone of his noble status; it would've breached some ancient aristocratic code of behavior.
The Duke's words are indeed ironic. For though he never stooped to discuss his wife's annoying habits with her, out of a sense of propriety, he nevertheless did stoop to killing her, violating every decent moral standard there is. For a haughty nobleman such as the Duke, social etiquette appears to be so much more important than morality.