The term is found in both Ireland and Scotland. It is sometimes used to translate the word sliocht, meaning seed, indicating the descendants of a person (i.e., Sliocht Brian Mac Diarmata, the descendants of Brian Mac Dermot).
Sil was used within the context of a family or clan, all who bore the same surname, as a manner of distinguishing one group from another. For example: a family called Mac an Bhaird (Anglicized as Ward) might be divided into septs such as Sil Sean mac Brian, Sil Conchobhar Og, Sil Sen Conn, Sil Cu Connacht. All of these individual lines might further sub-divide into still more septs, which in turn sometimes led to a new surname, and/or the emergence of the family considered a clan in their own right. This type of sept was normal in Scotland.
In Scotland, a sept is often a family that is absorbed into a larger Scottish clan for mutual benefit. For example, the Burns family sept was absorbed into the Clan Campbell. The Burns family, being very small and of questionable heritage, gained legitimacy and protection; the Campbell clan absorbed a potential rival for British affection in Scotland. Each Scottish clan typically has a number of septs, each with its own surname. Septs have rights to wear clan tartans although they often have tartans of their own. Related septs (see above) were also normal in Scotland.
Historically, the term 'sept' was not used in Ireland until the nineteenth century, long after any notion of clanship had since been eradicated. 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 MacLyslaght 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 centralized 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 Gaels, when speaking of themselves, employed their term 'clan'.