The London Daily Mail says the actress has been swept up in a "whirlwind romance" with Marcus Mumford, the frontman of current "it" band Mumford & Sons, who headlined the basement-set shindig in Tennessee.
"Marcus invited Carey to watch him play a secret show," says a pal. "She sat in the front row, and while he was singing Marcus looked down at Carey and she blushed. It was clear they had chemistry."
Mulligan, who's currently calling New York home, is apparently "besotted" with the London-based musician and has enjoyed a couple recent getaways with him in their native Britain.
"Carey's hoping they can spend more time together when Marcus flies back to America next month for a major tour with his band," continues the snitch. "He's hired a vintage train instead of a tour bus and has asked Carey to join him for the trip. It's early days, but they seem to enjoy each other's company."
Carey called it quits with Shia LaBeouf last October after a year of coupledom. In recent months, she's been briefly linked to fellow British thespians Tom Sturridge and Eddie Redmayne.
Mumford previously dated singer-songwriter Laura Marling. For those not up on your indie love triangles, Marling had been involved with Charlie Fink of Noah and the Whale. After their breakup, Fink chronicled his heartbreak on the band's 2009 album "The First Days of Spring."