Son of the Fenian Hero FINN-MACCOOL and a Goddess called SADB, who Finn ran across when she was a deer. The circumstances become increasingly bizarre. SADB was abducted by persons unknown whilst pregnant. FINN-MACCOOL ran across a small boy when out with his hounds one day, not realizing it was his son OISIN. Finn was a great one for cross-country running into strange situations with dogged persistence.
It now turns out that SADB had been the granddaughter of DAGDA, making OGMA, God of Eloquence, an uncle of OISIN. What with Finn having eaten the FINTAN, the Salmon of Knowledge, without seemingly having digested it, all the eloquence and wisdom got passed to OISIN who became top poet, songwriter and sensational lyre-plucker.
Then OISIN met NIAMH, a daughter of MANANNAN-MACLIR (how many daughters does this God have?) as she came trotting up on her steed with the golden mane and silver hooves. To his delight she invited him for a ride to "The Land of Promise"
We don’t know what went on there, but it went on for a long time, and OISIN not having been able to send a Postcard home with "having a Lovely Time — see you all soon’ scrawled on it, due to there being no postal service at all, thought he should just nip home to let folks know. NIAMH lent him her magic horse, but warned him he must not get off it, as it would return without him, and there would be no way back.
Back in Ireland, OISIN found he had done a Rip Van Winkle’ sort of thing, although he had not done so much sleeping, but you get the general picture — everyone he had known was either dead or departed. Helping some men to clear a boulder from a track, he slipped off the horse and thud. Then the thudding of hooves and the horse was gone. He tried to jump up and found that he himself was now incredibly old and frail.
It is said he was found and taken to tell his tale to St Patrick, because by now the new-fangled Christianity was claiming the land.