I think many people still consider the Irish a distinct racial group.
On the Jewish subject, the reason the Jews are considered an ethnic group are because they rarely ever looked for converts so many of the Jews in Europe are descended from the original 12 tribes of Israel, and not the Aryans or Hittites. In the old days(Babylon), the ethnicity was identified by the tribe, or the term "Hebrew" designated the ethnic group, and religion within an ethnic group was fairly constant. The Jews also lived in isolation from many people causing a distinct culture and ethnicity. Obviously, intermarriage was fairly common(post diaspora) as Jews now often look very European(or Semetic, Negro, or whatever.) Christians are not considered an ethnic groups because mass conversions caused Christians to exist in all previously defined racial groups.