The terms clan and sept have often been confused.  Ireland never had a well developed clan system like Scotland.   “Sept” is more appropriate as a collective term describing a group of persons or immediate ancestors who bore a common surname and inhabited the same territory.  Clan is often used today to  indicate the family group.  Source:  Edward MacLysaght, Irish Families, Dublin: 1991.  Bibliography  

Ireland was one of the earliest countries to evolve a system of hereditary surnames. They came into being fairly generally in the eleventh century, though some were formed as early as the year 1000. Brian Boru, high king of Ireland, who died at the Battle of Clontarf in 1014, is often erroneously credited with decreeing that the use of surnames should become a requirement among his subjects. In fact the system developed spontaneously in Ireland, as it did elsewhere, as a result of the need for personal identification in an increasing population.

With few exceptions, your current surname may not be close to your historical surname.    Many times, census takers and ship’s clerks spelled your family name phonetically.  Some examples below.