William Gropper had a lithograph series entitled American Folk Heroes.
 One of the lithographs is: Finn MacCool.  Who was FinnMacCool or what
was Gropper using this character to express

Hi mandolin3,

Thank you for this question. Finn MacCool stories came to America with
the Irish immigrants and Irish mythology is one of my favorite
subjects, so, here goes:

Gropper was probably using Finn MacCool to express justice, honor & chivalry.

Finn MacCool also known as Fionn mac Cumhail is a legendary Irish
leader and folk hero. He was the leader of a band of professional
fighters called the Fenians or Fianna.
His exploits are recorded in long narrative poems by Ossian and in many ballads, 
called Fenian ballads. 

You can read about him at FinnMacCool.com
http://www.finnmaccool.com/WhoWasFinnMacCool.htm

Here is an excerpt from FinnMacCool.com

Who was Finn MacCool? 

"If Ireland was Avalon, Finn MacCool would be Arthur....

Fionn mac Cumhail (Finn MacCool) was the greatest leader of the
Fianna; the military elite of ancient Ireland responsible for guarding
the High King.  The Fianna were founded in 300 B.C. by the High King
Fiachadh (fee-a-kuh).  Until Fionn mac Cumhail implemented a code of
honor among them, the Fianna had a reputation of being a somewhat
unruly bunch of men who considered themselves, to some small degree,
above the law, due to their position of power.  Fionn challenged the
Fianna to become champions of the people; to make of themselves models
of chivalry and justice that others may aspire to.  The tales of the
Fianna are argued to be the basis of those of the Knights of the Round
Table of England, with Arthur as their leader as Fionn was leader of
the Fianna.  He was also father of the great poet Oisín (o-sheen).

Here are a few more links you may want to check out:

Who Was Finn MacCool 
http://www.mth.kcl.ac.uk/~tcoolen/finn/maccool.htm

Finn mac Cumhail 
http://www.encyclopedia.com/html/F/FinnmacC1.asp

Ossian
http://www.encyclopedia.com/html/O/Ossian.asp

~wlk115

Search terms used:

Finn MacCool
Fionn mac Cumhail
Ossian

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Clarification of Answer by wlk115-ga on 01 May 2004 16:35 PDT

(Sorry, missed part of a paragraph when I was copy and pasting)

Gropper was probably using Finn MacCool to express justice, honor & chivalry. 
He was a favorite folk hero among the Irish miners who were used badly
by the mine owners and over the years his appearance took on the
trimmings of their new life in America (miner?s clothes, pick, &
shovel, etc).

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Request for Answer Clarification by mandolin3-ga on 02 May 2004 08:30 PDT

Your answer is great, but I did receive it twice.    Mandolin3

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Clarification of Answer by wlk115-ga on 02 May 2004 21:45 PDT

Sorry. That happened because I added the clarification. It sent the
original answer then sent the clarification.