Tuan mac Cairill was a wealthy warrior-prince who lived near Magh Bhile (now Moville near Newtownards in County Down) in the sixth century. In the immediate vicinity also resided an abbot by the name of St. Finnen. The saint had heard that Tuan was well-versed in the history of Ireland, and went to visit him one day in order that he might take down some of the man's knowledge.
When Finnen arrived at Tuan's door, he was refused admittance (relations between the Pagans and the Christians tended to be rather strained). Finnen knew that allowing a man to starve on your doorstep was a breach of the near-sacred Irish practice of hospitality, and so he sat down upon Tuan's entryway and fasted until the warrior finally invited him in. The stubborn admire the stubborn, as the saying goes, and the two visited together and became friends.
Finnen returned to the abbey and praised Tuan as a fine man and a great teller of the old stories of Ireland. Not long after this, Tuan paid a visit to the abbey. They asked him to tell them of his lineage and ancestry, that they might commit it to record. Tuan named himself a man of Ulster, Tuan son of Cairill, but then went on to say that on a time he was called Tuan son of Starn, who was brother to Parthalán. Since Parthalán and his people were long since extinct, the monks wondered at this and bade Tuan to tell his tale.
No explanation is given for Tuan surviving the plague that killed the rest of the Partholonians, other than Tuan's own assertion that "there is never a slaughter that one man does not come out of it to tell the tale." Tuan was then the only living man on Ireland, and spent the next thirty years wandering the island and dwelling in dead men's houses. At the end of this time, we have the arrival of Neimheadh and his people on Ireland.
By this time Tuan was, in his own words, "long-haired, clawed, decrepit, grey, naked, wretched and miserable." He stalked Neimheadh from afar, careful not to be seen by him. He tells that one night he laid down and when he arose the next morning, he was no longer a withered old man, but a virile young stag, burning with life. He was the king of all the deer of Ireland, and he watched the Nemedians blossom, grow and be driven away again by the Fomorians.
By this time, Tuan had become an ancient stag, grayed and grizzled with age, and he lay down at the entrance to a cave and underwent another reincarnation, this time into a wild boar. Again he was young and strong, and king of the boar-herds of Ireland. Next, Tuan tells of the landing of Semeon, grandson of Neimheadh, and his people the Fir Bolg. And again he becomes old, and again he returns to Ulster because, as he said, "it was always there that my transformations took place, and that is why I went back thither to await the renewal of my body."
This time, he was reincarnated as a great eagle of the sea. His youth on him again, he watches the Tuatha de Dannan arrive and conquer the Fir Bolg, and the Milesians arrive and conquer them in turn. Again age overcame him, and he returned to Ulster and slept, only to awake in the form of a salmon. He enjoyed his freedom among the water for many years until at last he was captured by a fisherman and taken to the stronghold of Cairill, the chief of the area. Cairill's wife then ate the salmon in its entirety, and his spirit passed into her womb.
And so Tuan was born again into the body of a man, and yet he retained memory of all he had seen and known through his many existences. The monks recorded his tale, and his accounts of the peoples that had populated Ireland in its ancient past. An interesting aspect of this story is the fact that while it revolves around a belief in, or at least an acceptance of, the very Pagan and very Celtic concept of transmigration of the soul, it is faithfully recorded and retold by Christian monks, whose religious beliefs run directly opposite. This practice of acceptance, which was markedly different from other Christian efforts at conversion (eg, The Spanish Inquisition or The Crusades), allowed the monks gradually into the confidence of the Celts, and they were thus able to convert a fierce warrior race with essentially no bloodshed.
On a bibliographical note, the quotes in this write-up are taken from R. I. Best's translation of "The Irish Mythological Cycle" by d'Arbois de Jubainville.