This just in from BBC News:
“Shirley MacLaine is joining the U.K. series for its upcoming third season. The Oscar winner’s addition makes her the second American cast member (and character) on the British period drama.
She’s set to play Martha Levinson, the American mother of Lady Grantham (Elizabeth McGovern), and joins the cast as they begin shooting the new season next month.”
Read more at my source: BBC