Perhaps his most significant role in 2004 , however , was Albus Dumbledore , Hogwarts ' headmaster in the third installment of J. K. Rowling 's franchise , Harry Potter and the Prisoner of Azkaban , taking over from fellow Irish actor Richard Harris , who had died of Hodgkins disease . Perhaps his most significant role in 2004 was Albus Dumbledore , Hogwarts 's headmaster in the third installment of J. K. Rowling 's franchise , Harry Potter and the Prisoner of Azkaban , taking over from the late Richard Harris .