O’Clery was the first known Irish surname. This was the earliest for it was written that Lord of Aidhne, Tigherneach Ua Cleirigh passed away in the County Galway in 916 AD.
The truth is, O’Clery might possibly the earliest surname that has been recorded in all of Europe.
Until the 10th century in Ireland, the surnames weren’t passed down. Rather, what happened was, the surnames were based on someone’s father or also called as patronymic. A person can be identified by the name given to him or her including the word “mac” or which means “son of” and then followed by the father’s name.
You Better Get Used to it
So as a quick example, John mac Colum means John, son of Colum. Mark may later on have a family and have a son and his son may be Chris. So it would be Chris mac John or Chris son of John. As for female counterpart, the word “mac” will be replaced to “nic” or short term for Irish iníon mhic.
Then again, the prefix “o” is used occasionally to replace “mac” and intended to be “grandson of” or “descended from”. From our previous example “Colum”, if he is a known personality, then his grandson may have gone by Chris o Colum.