Fostering was also widely practiced where sons and daughters were sent to live with a more prosperous family and sometime the foster children would take the surname of the foster family.  Most often the family doing the foster would receive a payment for that service, so in many ways it was very much like our modern practice of sending a child off to college.  Fostering was also used as a form of debt payment, children would be given over to the foster family to discharge a debt.  Fostering more than any other possibility may account for the mix in DNA within surnames.  Children born to daughters normally were given the surnames of their fathers but once the children reached fostering age around eight years old, a fostering with more well to do related families could occur.  In this example the son of an O'Kelley might be fostered by his maternal family because of their wealth and status and that son could take on the surname of his fostering family providing us with an explanation as to how an O'Kelley Y DNA could become a Conley or Farrell.

There are other customs as explained in "The Short History of Ireland" by Dr Johnathan Bardon that complicates genealogy.  The Irish followed the teachings of the Old Testament by having a primary wife and secondary wives and concubines.  An Irish husband could send his wife and her children back to her family and the children would often then take the name of their maternal grandfather.  This book also tells about "fostering" a practice where the Irish may send their children to live with a family for any number of reasons and those children would sometimes become the children of that family taking the name of the family.  Another custom that may seem shocking is there are several accounts in this book about the Irish women's dress.  They would wear short smocks and nothing else.  The Spaniards wrote in their letters home about the beauty of the Irish women and how immodest they were about their private parts being exposed.  I am certain this too accounts for a great number of genealogy surprises. 

Nancy Stenson in her book tiled "Basic Irish"  provides additional explanations why surnames can not be relied upon.  The Irish would sometimes take the maternal name if the maternal relations was powerful , wealthy, or famous and when there were two of the same name in a village, they would use a local name that identifies the person with their fathers or mothers and not their family.  For example if Farrell Ua Ceallaigh and Patrick Ua Ceallaigh each had a son name Shaun Ua Ceallaigh, in the village they would be known as Shaun MacFarrell and Shaun MacPatrick and in some cases these names stuck and were passed down and as grandsons they would appear as O'Farrell and O'Patrick.

Another activity that is not talked about is the Irish as was most cultures were a warrior race that took what it wanted from its neighbors.  This is part of the reason England and Ireland dealt so harshly with each other, the Irish raided England and took both slaves and property killing those who opposed them.  Because our common ancestor was probably a warrior and he impregnated many conquered woman as was the custom; the taking of the spoils of war, it too makes it possible that many Kelly males are living under different surnames.  The mistake that is commonly made by researchers is we tend to think of our ancestors like we think of a favorite modern grandfather and that just isn't the way our ancestors were.  They lived in a dog eat dog world and were often cruel and violent.  If you do genealogy you may want to keep this in mind.