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.