None other than Cary Grant himself called her the finest comedienne he ever worked with, a mistress of comic timing and touch. Her lyrical inflections and very musical voice (she was a classically trained vocalist) lent a merry spark to every film she ever made. Mired in heavy melodramas and tearjerkers in the early '30s, Irene's comic talents leaped from the screen for the first time in the sublimely silly "Theodora Goes Wild" in 1936. From that point on, she was widely recognized as one of the screen's finest and funniest comediennes, and her particular brand of subtle hilarity lit up the screen in films like The Awful Truth (1937), the original Love Affair (1939), and My Favorite Wife (1940).
You truly have to see and hear Irene in action in order to appreciate her. Words cannot convey the humor of that particular hesitating, under-her-breath laugh, or the insinuating expressiveness of her very mobile face and eyes. As Grant noted, she had an uncanny sense of comic subtlety and timing, and even when the material was less than sparkling (as in Joy of Living (1938)), her talent could still salvage most films.
If you've never seen an Irene Dunne film, I would highly recommend that you do!