I don't vouch for the accuracy of this account of the term Mickey Finn, but offer it for whatever it's worth:

http://thebachelor.crosswinds.net/mfinn.htm, beginning:

ONCE UPON A TIME, back in the 1880's of the last century, there was a saloon-owner in Chicago whose name was Michael Finn. No, Mr. Michael Finn did not invent the Mickey Finn. The guy who ran the saloon right across the street from Mr. Finn's did that.

Edit: according to this account, the original Mickey Finn did render the victim unconsious. It neutralized an obnoxious drunk by a different means: strong purgatives. [bill, am I being suitably euphemisic?]