Was Elliott Ness a FBI agent, IRS agent, ATF agent or something else when he went after Capone?  

Ness was in charge of the Cleveland police force. Believe the Untouchables was from a book he had written. google.com is a good place to look things up

I already checked on the googles and couple of sites I looked at described him as the "Federal" agent, Cleveland's Safty Director, "Revenue" agent, etc.  

Ness was in the U.S. Treasury Department's Prohibition Bureau.....ATF

BarracudaBob is correct.

Ness was a tresury agent. I think this was kind of a pre-ATF.

He is absolutely correct, and where did you get that answer from? any site I could reference?

I learnt it in skool. Honestly, I have no clue where you could actually find info on Ness...but it's safe to say that if you type in Eliott Ness into your search engine, you'd probably get a zillion hits on it. Maybe sites like the ATF and Treasury Dept.

Good Luck

cuz it says so in [i]The Untouchables[/i] movie with Kevin Costner, Sean Connery, and Andy Garcia. [;D]  seriously, that's why they got him on income tax evasion.  not even the ATF gives a crap about income tax evasion.

[url]http://arachnid.pepperdine.edu/goseweb/u.htm[/url]
Also....
1930 - Secret Six of the Chicago Crime Commission (founded 1919 because of the labor violence during WWI) decide to take action Feb. 5 against Capone - joint local-federal effort led by Alexander Jamie, chief agent of the Prohibition Bureau in Chicago (Elliot Ness was his bother-in-law and a minor figure in the crackdown, but later exaggerated in the book The Untouchables written by Oscar Fraley and Ness); helped by Elmer Irey and Frank Wilson of the Treasury Dept. and the infiltrator Pat O'Rourke; the Secret Six are Frank Loesch (leader of the Chicago Bar Association), Samuel Insull (owner of Commonwealth and Edison Power Co.), Julius Rosenwald (chairman of Sears), Robert I. Randolph (president of Association of Commerce), Edward Gore, George Paddock.

[url]http://history.acusd.edu/gen/classes/20th/FBI.html[/url]

(Edited to add additional reference)

Elliot ness was a revenue agent until Prohibition ended when he returned to the Cleveland PD. There was some kind of question about an incident that occurred while he was a Cleveland PD Officer and it haunted him for the rest of his life.

Revenuers were a branch of the Treasury as in IRS. A branch of the IRS was set aside specifically under the GCA of 1968 to fight Gun Crimes. In 1972 it was renamed the BATF and became the Law Enforcement Barnch of the IRS with the exception of Income Taxes which were still handled by the IRS.

The incident that haunted Ness in Cleveland was the Torso Murders. Between 1934 & 1938 there were at least 12 victims. Ness focused more manpower on this investigation than any previous police action in Cleveland but no one was ever arrested or officially identified as the killer.
I'm not too far from Cleveland so it has always interested me and there is a new book out called In The Wake Of The Butcher by James Jessen Badal. I just picked it up and haven't read it yet but it's supposed to go into great detail about Ness.