Thanks for your reply...
We are talking 2nd cousins, we are all the grandchildren of 8 original brothers and sisters.
We all match regardless I presume because of the mother of these 8 children, we have matches in common across the board for the mothers family, as in all the relevant cousins we can see have matches to the mothers family but the one person I find with the Mahon surname from another Mahon as I describe below doesn't match any of them.
Having researched this for many years, it lead me back to Killeen, Meath in the 1830's and to what I on paper I made the assumption were 2 brothers (John & Cornelius) marrying within days of eachother - this was a hunch at the time based on shared witness names and their location, etc - I then found Cornelius going to the US - I was able to find a descendant of him and his wife who I contacted. This is the person I mention in the thread that told me he had done Ancestry DNA test and a huge breakthrough when we do in fact match!!
I have since matched with every relative on his side that has taken the DNA test, as has my sister.
The obvious reason for the lack of match would be that MY grandfather is the odd the odd one out and was fathered by someone else, the child of a family member, etc - but it's his descendants e.g. me and my sister who match the people in America and they are the people who share the same MAHON surname and who we have matched them to in our search. So even prior to the DNA test results we believed they were related.
Looking at it does everyone reading this believe that the disconnect could only have happened with the parents of these 8 children?
I can't see even if my grandfather had different a different father how his descendants can share a connection with someone who is also a direct descendant of a Mahon, who we know is from the same area and we believed was a relation before the DNA almost 200 years ago.
Any other views would be great and I look forward to your replies