The following comes from the Patron Saints Index:
Daughter of the Duke of Clarence. Niece of King Edward IV and King Richard III of England. Married Sir Richard Pole in 1491. Mother of five, one of whom became a cardinal. Widow. Unofficial ward of King Henry VIII, who made her Countess of Salisbury and governess to Princess Mary, daughter of Henry VIII.
When she opposed Henry’s plan to marry Ann Boleyn, she was driven from court and received the king’s disfavor. When her son, Cardinal Reginald Pole, wrote against Henry’s presumptions to spiritual supremacy, the king decided to crush the family. Two of Margaret’s sons were executed in 1538 for the crime of being the brothers of Reginald. The elderly Margaret was arrested soon after, falsley charged with plotting revolution; in 1539 she was sent to the Tower of London where she spent her remaining two years. In 1541, at the outbreak of an actual uprising, Margaret was summarily executed with trial as a precaution. Martyr.
To find out more about the Blessed Margaret Pole film here.