NIAMH AND OISÍN IN THE AGALLAMH

This is an older version of the Laoi Oisín ar Tír na n-Óg, which features a far less otherworldly story of the love affair between Oisín and Niamh, the latter being here a daughter of Angus tíreach, a king of Munster.
The translation is that of Standish H. O'Grady, and the tale appears in volumes 1 (the transcription of the Irish text) and volume 2 (the translation) of Silva Gadelica, published in 1891 by Williams & Norgate. The translation is on pp.178-9 of volume 2.
The text originated in theAgallamh na senórach ("Colloquy"), which appears in the Lebar Még Charthaig, f. 159, col. 1.
Incidentally, a story of an otherworldly interlude on p.223 features Oisín, in the company of Finn, Oscar, Diarmaid ó Duibhne and mac Lughach, in association with a "worshipfully [...] soft girl [...] yellow-haired, of marriageable age," and "a translucent crystal seat."

Then Colman questioned Caeilte: "what is the cause that the name of tipra an bhantrachta or 'the well of women' is given to this well close against the loch?"

Caeilte answers that: "it was Niamh, daughter of Angus tíreach king of Munster, that from dún na mbare in the province of Munster eloped with Finn's son Ossian and came to the well; here he was with her for six weeks, enjoying the hunting and venery of Ulidia; the damsel too with her thirty women used to come every morning, and in this blue-surfaced water they would wash their faces and their hands.

"That his daughter was stolen away with Ossian lay very heavily on the king of Munster; both provinces of Munster were mustered by him: five hardy battles equal in bulk, and in pursuit of the Fianna they came hither. Just then Niamh washed herself at the well, and she saw the five battles on the tulach right over her.

"'Alas for it,' the young woman cried: 'and happy she that had died, or been slain, ere her guardian, her father, her three brothers and Munster's nobles had seen her thus!'

"She laid her face on the ground and, with the thirty her companions, died; as for her, her heart as a lump of black blood passed from her mouth, and hence it is that from that time to this cnoc an áir or 'the hill of slaughter' is this tulach's name."

Then Caeilte uttered: -

In this hill lives the queen...

"When both provinces of Munster saw the woman-folk's death their king said: 'an evil undertaking hath been this of Ossian's and of the Fianna's against us!' and he enjoined his she-runner Muirenn daughter of Muiresc to seek out Finn and challenge him to battle. The runner went her way to ráth chinn chon or 'rath of the wolf-dog's head' in Dalaradia, where the Fianna were. Finn sought her tidings, and she told him the errand on which she came.

"'Until this day,' said Finn, 'it has been a rare thing to challenge me to battle! go, Garbchronan, summon the Fianna to the fight.'

"He went out and, standing over the Fianna's leaguer, emitted three wrathful larum-cries which were heard in the heart of their camp; and the Fianna answered, for they knew that some great motive urged him to haste. They rose therefore and stoutly arrayed themselves in order of war; then of Finn enquired the cause of battle, and he told it them.

"Now said Fergus True-lips to Finn: 'Fian-chief, for giving battle to the king of Munster in the matter of his daughter whom thou hast slain thou hast not right on thy side.'

"Then by Finn and the chiefs of the Fianna a course was determined on, pursuant to which he said to Abartach's daughter Smirgait: 'tell Angus tíreach and Munster's nobles that I will pay them the award of Cormac grandson of Conn, of Eithne ollardha daughter of Cahir More, and of Cithruadh son of Fercaecait.'

"The runner departed and delivered what she had to say.

"'It shall be accepted,' Angus said, 'if bondsmen are sureties for its fulfilment be put in.'

"'What sureties requirest thou?'

"'The son of him that hath done me wrong: Oscar son of Ossian, and Ferdoman son of the Daghda's son Bodhb Derg, and Dermot son of Donn son of Donough.'

"Finn yielded that and both parties repaired to Tara, where the judgment given them was this: the girl to be raised out of the tulach in which she lay, and put into scales; her own weight of gold and again her own weight of silver to be given to the king of Munster in eric of her; a separate eric to be paid for every chief or chieftain's daughter that perished there.

"'Fianna of Ireland, how shall we apportion such eric?' said Finn.

"They answered: 'one-third from clan-Baeiscne; from us the Fianna, two.'

"And this, Colman," ended Caeilte, "is the only eric that ever Finn allotted among the Fianna."