The Girls star gave birth to a baby boy on November 20, People reports.
Kirke and her husband Michael Mosberg are thought to have named their new addition Memphis.
The couple, who wed three years ago, announced in June that they were expecting their second child.
They are already parents to a 2-year-old daughter, Rafaella Israel.
Kirke will return for the second season of Girls, which premieres on Sunday, January 13 at 9pm on HBO in the US and a day later at 10pm in the UK.
Watch a trailer for the second season of Girls below: