Byrne, 33, is best known for her work on the TV legal drama "Damages" and in the big-screen comedy "Bridesmaids."
Cannavale, 42, is a cast member of Hob's "Boardwalk Empire" and can now be seen in the Broadway revival of "Calgary Glen Ross."
"They've been dating for a few months," an insider told Usmagazine.com. "I think Rose is a good match for him."
NBC reportedly holds celebs hostage to Jimmy Fallon's show
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