Prior to the development of hereditary surnames in Ireland, there was a sept system by which families were divided into broad clans or tribes. These were usually based on a common descent from a particularly notable ancestor. For example, the septs who all claimed descent from the famed 4th century warrior king Niall of the Nine Hostages were collectively known as the Ui Neill, or the Hy Niall. Other particularly distinguished groups of ancient septs included the Ui Fiachra, the Ui Maine (also called the Hy Many), the Cinel Eoghain, the Clann Cholgain, the Corca Laighe, and the Dal Cair (also known as the Dalcassians). The use of surnames gradually rendered the sept system obsolete.
The original tribe name was occasionally perpetuated as the hereditary surname of the senior family of the sept, but this was unusual. Instead, hereditary surnames were developed from locations, nicknames, occupations, or family relations. As time went by, sub-septs gradually formed, splitting off from the major sept and taking their own surnames. This was done for a number of reasons, especially to further distinguish between a number of individuals with the same names. However, change also arose from regional variations and clerical errors.
- ^ Swyrich, Archive materials