Dermot Mulroney was born on October 31, 1963 in Alexandria, Virginia. His mother, Ellen, was a homemaker and amateur actress originally from Manchester, Iowa, and his father, Michael Mulroney, was a law professor (and amateur racecar driver) at Villanova University School of Law, originally from Elkader, Iowa. He has a sister, Moira, and three brothers, Conor, Kieran (who is also an actor), and Sean. Moira formerly worked in the Philadelphia DA's office as an assistant district attorney, and later at WXPN, an FM radio station in Philadelphia, but now works elsewhere in the non-profit industry. Some of Mulroney's ancestors emigrated from County Donegal and County Mayo in Ireland during the Irish Potato Famine; he is of seven eighths Irish and one eighth German ancestry. Mulroney attended Maury Elementary School and played cello in school and city youth orchestras, as well as acted in children's community theater. Mulroney attended the Interlochen Arts Camp as a cellist. He later attended T. C. Williams High School (grad. 1981) as well as Northwestern University (grad. 1985) in Evanston, Illinois, where he was a member of the fraternity of Phi Gamma Delta. He is known for his roles in the films Young Guns, Samantha, Where the Day Takes You, My Best Friend's Wedding, The Last Outlaw, About Schmidt, and August: Osage County. Mulroney played the main antagonist of NBC's Crisis, Francis Gibson.

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