Announcement: "[somebody1] with [somebody2]." Could've been "[Musical Guest] with [House Band]," or "[Musical Guest] with [Collaborative Artist Who Also Appears on Album]," or "[Musical Guest] with [Special Guest Only for this TV Appearance]." Somebodies 1 and 2 may have been solo artists, or they may have been band names.
Performers: Male lead singer with hillbilly beard. Thin female vocalist with straight shoulder-length brownish hair. Horn section (house band?). Other band members--guitar, etc. There may or may not have been a fiddle player.
Song: I want to say kind of country/Irish/folky.
Lyrics: I think the guy mentioned an ex who's "with another man" now, but he hopes that she's happy. Or something. I had the volume really low so as not to bother my landlords.
I'm almost embarrassed to post information this vague on the "identify this song" threads at my usual popcult forums, so all I can do on my own is look up all the musical guests on all the late-night talk shows for that week.
NBC: The Tonight Show with Jay Leno, Late Night with Conan O'Brien, Last Call with Carson Daly.
ABC: Jimmy Kimmel Live.
CBS: The Late Show with David Letterman, The Late Late Show with Craig Ferguson,
1/18: Teagan & Sara, 1/19: Metric, 1/20: The Fray, 1/24: Marty Stuart, 1/25: Yellowcard, 1/26: Norah Vincent, 1/27: Lewis Taylor.