The legend of Tuan Mac Carrell is found in an 11th century manuscript called Lebor na hUidre (The Book of the Dun Cow). Tuan tells a Christian monk that he was born 2,000 years earlier and witnessed many of the waves of invaders who came to ancient Ireland — the Nemedians, Firbolg and the Tuatha de Dannan. As an old man he crawled off into a cave and fell asleep and when he awoke he had been reborn as a vigorous young stag. The process repeated itself each time he became old and he was reborn variously as a wild boar, an eagle and eventually as a salmon. However, during his existence as a salmon he was caught and eaten whole by the wife of a chieftain called Carrell and passed into her womb to be reborn again as Tuan Mac (son of) Carrell. The myth clearly suggests that there was a belief in reincarnation among our Irish ancestors.
So, if Tuan was reincarnated over a 2,000 year period up until the early Christian era in Ireland (circa 600-800AD) who is to say that the process didn’t continue? That leaves the possibility that someone could still be running around today claiming to be the reincarnation of the ancient chieftain (although they fail to mention the bit about also being a fish). It is a scenario that was just crying out to be turned into a gory police procedural story with (at least in my head) a soundtrack by Horslips.