McMahon DNA Project
An odd thing happened when I had my Y-DNA tested with FamilyTree DNA.All of my closest matches were McMahon's from Monoghan County, Ireland. My family is easily traced to 1800 when they came from Ireland to the U.S. So it would appear that sometime earlier through adoption, etc. there was a surname change from McMahon to Courtney.
DNA is a wonderful tool so please if you have any interest checkout the McMahon Surname Project at ftDNA.
http://www.familytreedna.com/public/mcmahon/http://www.familytreedna.com/public/mcmahon/