A smallish tribe of Celts who settled in the Lancashire area of northern England after migrating from Europe, and coexisted with the Roman occupation of Britain. There is some evidence that the tribe actually straddled the Irish Sea with branches in Ulster as well as Britain.
There is some uncertainty about the derivation of their name. It could either be linked to the ancient Irish culture-hero Sétanta "he who knows the way", later dubbed with the epithet Cú Chulainn -"Hound of Culann" by Cathbad the druid after killing a fierce hound and offering to guard Culann the smith's house in its stead. The alternative is that the term "Setantii" was a romanisation or corruption of a Celtic demonym denoting "dwellers on the water", possibly referring to the tribe's relationship to the Irish Sea.
They are counted among the tribes that composed the Brigantes federation, which held a vast tract of land in defiance of Roman forces up until sometime in the mid 2nd century. Roman sources indicate a fairly decisive defeat between 150-200AD.
The Setantii feature heavily in a novel I'm working on that is partly set in Roman Manchester. I think they were probably pretty cool people.