Based on the provided source material, there is no information available regarding a city named Mullen in the United States. The name Mullen is identified as a surname of Irish origin.
Historical records indicate that the O'Mullens were a chiefly family belonging to the Uí Mháine tribe. These people originated from the Dumnonii or Laigin, who were part of the third wave of Celts to settle in Ireland during the first century BC.