Geanain, from whom the surname Gannon has ultimately been derived, was a chieftain of one of the five Irish provinces that arose circa 500 B.C as a sanctuary from Roman suppression. That essentially means Gannons are part of the original Irish "founding fathers" and blue-blooded to boot, at least so far as is indicated by the name.
Geanian's province was named after his daughter, Olnemacht, and through some 'revisions' of the name over the years, it came to be known as Connacht, the modern-day province of Ireland. It is there in today's Connacht that the Gannon family originated over 2 millenia ago. Many have remained, with a significant proportion having taken root in the county Mayo; unsurprisingly then, a large number of Gannons can trace their ancestors back to Co. Mayo.
Today, Gannon is spoken as "O Geanain" in Ireland, still retaining the original meaning mentioned above. More formally, Gannon is the anglicization of Mag Fhionnain. The "Mag" indicates "son of" (as in, MacGyver was 'the son of Gyver'), and this is expressed, too, by a prefixed "O," as seen with the aforementioned "O Geanian." To end a much-too-long explanation, a more "accurate" anglicization of Mag Fhionnain and one closer to the spoken Irish "O Geanain" would be "O'Gannon." All that "Mag" stuff aside, "Fhionnain" is derived from the Gaelic word "fionn" meaning "fair." I apologize if this disappoints, but it does not speak of one who is righteous but rather one who is fair-skinned. Those familiar with the history of skin tone perceptions in Western Europe (all of us, to be sure) will recognize that to be fair-skinned was a quality normally reserved for the royalty. Whether it was because they could wash more easily or because they were more healthy depends likely on which historian of these sorts of things you ask, but it seems to be of little consequence nonetheless.
I've also gathered that the family motto is "with sword and valor." Discussion on the topic is sparse, but all I've seen thus far has been in affirmation of the above. The coat of arms (or at least one of its many variations, this being the most common) can be found at
http://www.heraldry.ws/htm
l/gannon.html, and I kindly suggest it be made the image 'thing' for this group.
The information above was compiled mostly from:
http://www.shrule.com/_shr
ule/_display.php?fid=901&p
id=931 http://www.touchtel.ie/gen
ealogy/scripts/Family.asp?
FamilyID=142http://www.irishsurnames.c
om/cgi-bin/gallery.pl?name
=gannon&capname=Gannon&let
ter=g&user=