Irish clans are traditional kinship groups sharing a common surname and heritage and existing in a lineage based society prior to the 17th century.
The Irish word clann is a borrowing from the Latin planta, meaning a plant, an offshoot, offspring, a single child or children, by extension race or descendants. For instance the O'Daly family were poetically known as Clann Dalaigh, from a remote ancestor called Dalach.
Clann was used in the later Middle Ages both to provide a plural for surnames beginning with Mac meaning Son of. For example, "Clann Cárthaigh" for the men of the MacCarthy family and "Clann Suibhne" for the men of the MacSweeny family. Clann was also used to denote a subgroup within a wider surname, the descendants of a recent common ancestor. For example, the Clann Aodha Buidhe or the O'Neills of Clandeboy whose ancestor was Aodh Buidhe died in 1298. Such a 'clan' if sufficiently closely related, could have common interests in landownership, but any political power wielded by their chief was territorially based.
From ancient times Irish society was organised around traditional kinship groups or clans. These clans traced their origins to larger pre-surname population groupings or clans such as Uí Briúin in Connacht, Eóghanachta and Dál gCais in Munster, Uí Neill in Ulster, and Fir Domnann in Leinster. Within these larger groupings there tended to be one sept (division) who through war and politics became more powerful than others for a period of time and the leaders of some were accorded the status of royalty in Gaelic Ireland. Some of the more important septs to achieve this power were Ó Conor in Connacht, MacCarthy of Desmond and Ó Brien of Thomond in Munster, Ó Neill of Clandeboy in Ulster and MacMorrough Kavanagh in Leinster.
The largely symbolic role of High King of Ireland tended to rotate among the leaders of these royal clans. The larger or more important clans were led by a Taoiseach or Chief who had the status of royalty and the smaller and more dependent clans were led by Chieftains. Under Brehon Law the leaders of Irish clans were appointed by their kinsmen as custodians of the clan and were responsible for maintaining and protecting their clan and its property. The clan system formed the basis of society up to the 17th century.
"Sept" or "clan"
Scholars sometimes disagree about whether it is better to use the terms "sept" or "clan" when referring to traditional Irish family groups. Historically, the term 'sept' was not used in Ireland until the nineteenth century, long after any notion of clanship had been eradicated. It is often argued that the English word 'sept' is most accurate referring to a sub-group within a large clan; especially when that group has taken up residence outside of their clan's original territory. (O'Neill, MacSweeney, and O'Connor are examples.) Related Irish septs and clans often belong to larger groups, sometimes called tribes, such as the Dál gCais, Uí Néill, Uí Fiachrach, and Uí Maine. Recently, the late Edward MacLysaght suggested the English word 'sept' be used in place of the word 'clan' with regards to the historical social structure in Ireland, so as to differentiate it from the centralised Scottish clan system. This would imply that Ireland possessed no formalised clan system, which is not wholly accurate. Brehon Law, the ancient legal system of Ireland clearly defined the clan system in pre-Norman Ireland, which collapsed after the Tudor Conquest. The Irish, when speaking of themselves, employed their term 'clan' which means "family" in Irish.
The end of the clan system
In the 16th century English common law was introduced throughout Ireland. Together with a centralised royal administration in which the county and the sheriff replaced the 'country' and the Clan Chief.
When the Kingdom of Ireland was created in 1541, the Dublin administration wanted to involve the Gaelic chiefs into the new entity, creating new titles for them such as the Earl of Tyrone, or Baron Inchiquin. In the process they were granted new coats of arms from 1552. The associated policy of surrender and regrant involved a change to succession to a title by the European system of primogeniture, and not by the Irish tanistry, where a group of male cousins of a chief were eligible to succeed by election. This change to the inheritance system was also taken up by the Scottish clans in the 17th and 18th centuries. .
The early 17th century was a watershed in Ireland. It marked the destruction of Ireland's ancient Gaelic aristocracy following the Tudor re-conquest and cleared the way for the Plantation of Ulster. In 1607 the senior Gaelic Chiefs of Ulster left Ireland to recruit support in Spain but failed, and instead eventually arrived in Rome where they remained for the rest of their lives (See: Flight of the Earls). After this point, the English authorities in Dublin established real control over all of Ireland for the first time, bringing a centralised government to the entire island, and successfully disarmed the native clans and their lordships.
Later developments and "revival"
However, despite the loss of their traditional lands and forced emigration into the service of Catholic monarchs across Europe, the spirit of the Irish clans remained. The growing influence of the Gaelic League at the turn of the 20th century rekindled an interest in Gaelic culture and prompted a cultural revival.
In the 1940s Edward MacLysaght, the Chief Herald of Ireland, drew up a list of over 240 Irish clans. The first modern Irish "clans" were reformed in the latter half of the 20th century. Today such groups are organised in Ireland and in every continent around the world.
In 1989 an independent organisation, Clans of Ireland, was formed under the leadership of Rory O'Connor, Chieftain of the "O'Connor Kerry Clan", with the purpose of creating and maintaining a Register of Clans. A Register of Irish Clans can be consulted on its website.