The New York Times has an interesting story  about the late Cardinal John O'Connor's Jewish heritage:
In his 16 years as the Catholic Church's top official in New York, Cardinal John Joseph O'Connor was a staunch friend and defender of the Jewish people.
He spoke often about what he had seen at Dachau as a Navy chaplain. He used his platform as head of the Archdiocese of New York to support Soviet Jewry, and played a role in the Vatican's recognition of the state of Israel. Mayor Edward I. Koch, a Bronx-born Jew who worked closely with the cardinal, proclaimed that he loved him "like a brother."
Yet there was something Cardinal O'Connor apparently never knew: His mother was born a Jew, the daughter of a rabbi.
"The basic fact is, my mother was Jewish," said Mary O'Connor Ward-Donegan, the cardinal's 87-year-old sister. Observing the Jewish matrilineal tradition, she added, "That means my two brothers were Jewish, my sister was Jewish and I am Jewish. Of that I am very proud."