It would appear that the belief that Warden was Jewish is based on an erroneous statement in his obituary in the L.A. Times.
Obituaries, are sometimes erroneous. Wikipedia, is a terrific source but I am still not convinced he wasn't part Jewish.
Jack's not dead! Jack would never die without telling me, first!

In light of more recent inquiries in the years since I started this thread, it would appear that Jack Warden was not Jewish. See the discussion on his Wikipedia page: http://en.wikipedia.org/wiki/Talk:Jack_Warden. Wikipedia lists him as being of Pennsylvania German and Irish descent. It would appear that the belief that Warden was Jewish is based on an erroneous statement in his obituary in the L.A. Times.

Are you sure?
His original last name sure wasn't Irish.
He was Jewish and Irish. I am aquatinted with his niece. Her father I believe, was born in Sligo.
Jack's mother was Irish, Costello, was her maiden name (which is IRISH) and his father was German-Jewish. I don't know much more but you can also read his biography, it states he was of Irish and German2000-Jewish lineage.
I am not sure why that is so hard for people to accept. There are a LOT of Irish Catholic girls, who marry Jewish boys, and vice-versa, especially back east, New Jersey and New York. I believe Jack was born in New Jersey.
Really, not that uncommon.
Sean Penn's parents were Russian Jewish (Dad), and Irish Catholic (Mom).
Robert DeNiro's Mom was Irish (and maybe French, too) and his Dad was German & Italian (although, I don't know if DeNiros Dad was Jewish)
Matthew Broderick's Dad's family was from Donegal and his mom is Jewish.
What, can't you be serious, just for once in your life?

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Are you sure?
His original last name sure wasn't Irish.

Jack was Irish!

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Maybe you have "never heard of anyone associating the redhead attribute to someone of Jewish ancestory". Obviously you didn't know my grandmother & her three daughters & son who had naturally red (more auburn)hair! My natural hair color has some auburn highlights throughout. You're also correct that red hair is typically Irish-several of my friends with Irish heritage have red hair. Although this is really interesting & I've learned so much here; why are we all so concerned what his religion was? Remember that Jewish is a religion, even though you're comparing it to Irish, which is referring to a country. There are Jews from many countries, even China.

Well, arguably the Jewishest Jew, Woody Allen LOL is a red head so maybe a lot of people do consider it a Jewish attribute.
The revolution will now be televised. Sundays on Adult Swim.

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Costello is actually an Irish name. The only Italians I know of named Costello changed their names to that: Lou Costello (real name Cristillo) and Frank Costello (real name Castiglia). Other than their descendants, everyone named Costello is Irish.
The revolution will now be televised. Sundays on Adult Swim.

His mother's maiden name was Costello.. and I believe that is an Italian surname. (Might have been half Italian?)

Yeah, I thought that was going to be the O.P.'s final conclusion.
He even mentioned that "red hair can be a sign of Jewish ancestry."

He was Jewish.
http://www.jewishsailors.com/famous/

http://rightonpeachtree.blogspot.com/

Lebzelter is a term meaning "Baker of honey cake" (an occupation of beekeepers; the other occupation is pulling candles from the wax the bees made to store the honey) and is used by the Bulkes people people who were ethnic Germans living in a part of Hungary that became part of Yugoslavia. (They were removed from the area ethnically cleansed by the Red Army in 1944.) The Bulkes were a group of expatriate Germans known generally as Donauschwaben people as many originated from Schwabia*.
http://www.genealogienetz.de/reg/ESE/dschwaben.html

• "Schwabia is an old name for the land, people and culture of most
  southwestern Germany. Schwabia was bordered by the French on the west,
  the Alps on the south, the Bavarian Highland on the east and Frankish Germans
  on the north (refer to map.) Schwabia was once under the rule of a variety of
  medieval kings. The strongest Kingdon emerged as Wurrtenburg, a kingdom
  that existed until 1871 when it was incorporated into the United German Reich.
  "Schwabia or Wurrtemburg is a hilly and forested area with valleys of
  vineyards, Orchards and wheat fields. The fabled Black Forest boarders its
  western edge. The Schwabish-Worrtemburgers, are known for their thrift and
  craftsmanship. It is a heavily populated province with the well-known cities of Stuttgart and Ulm."

Lebzelter is Polish I believe. I would guess he is Polish/Irish. He has a lot of features that are common in both.

I think your guess is probably right. First name "John" is not commonly given to Jewish kids.