Although Fionn served as chieftain of the Fianna at the request of Cormac Mac Art, ruler of Tara, there came to be unrest between them because Cormac grew wary of the power of the Fianna thinking that they might one day oust him from his kingship.
This made Fionn very sad for he was extremely loyal and hated that there would be disharmony between himself and the King. He was getting on in years and feeling a bit lonely for his old age and felt it was time that he should take a new wife for his second wife Maighneis the mother of Fiachna had been dead for many years. When it was suggested that he should ask for the hand of Cormac's daughter Gráinne he felt very eager feeling that he could heal the rift that had formed between himself and the King, while at the same time gaining for himself a beautiful wife.
He therefore sent messengers to Cormac Mac Art to ask for the hand of his daughter. Cormac was agreeable to this plan for he would then turn a potential rival into a son-in-law. Gráinne herself though was not so willing for she thought Fionn was too old and that she might find someone who she could truly love. Her father would not listen to her misgivings and commanded her to do as she was told. So he sent back messages of friendship and acceptance of his offer to Fionn.
A great feast was then prepared at Tara, and people came from all five provinces of Ireland to grace the wedding of Fionn to Gráinne. On the night of the feast Gráinne sat pale and wordless beside Fionn until she caught sight of Diarmuid O'Duibhne among the guests and a blush of colour passed over her face and she asked his name.
Then taking the guest cup in her hands Gráinne moved among the crowd speaking to one or sharing a joke with another until she came to where Diarmuid O'Duibhne sat and whispered to him 'My heart is filled with longing for you' Diarmuid looked up at his lord's new wife startled and for a moment his eyes filled with reciprocal feelings but he refused her saying he would not betray Fionn.
Gráinne was not to be dissuaded however and she put a geis on him to accompany her out of the feast hall or leave Ireland as a dishonoured man. Then Diarmuid grew pale for no warrior could refuse such a request but he knew that once they left Tara there would be no safe haven from the reaches of Fionn and his men.
Because of the geis he had to do her bidding and so under cover of darkness while Fionn and all his men slept the two lovers crept out of Tara and fled away. Many times as they were running they heard the cry of the hounds and more than once Diarmuid was nearly caught by either Bran or Sgeolan the hounds of Fionn but each time he managed to escape.
The days grew into weeks and the weeks into months and still the pair fled from Fionn and the Fianna. Diarmuid had still not slept with Gráinne because he was trying to be loyal to Fionn. One day as they were running along Gráinne ran into a puddle and the water splashed between her thighs, she turned and mocked Diarmuid "See, Diarmuid even the water in a puddle of Ireland is braver than you!" This reproach stung Diarmuid's pride in his manliness and he then made love to Gráinne. Slowly Diarmuid began to truly love Gráinne. The elopement took its toll on Gráinne and she grew thin and brown and her once carefully dressed hair became entangled but she never flinched from any path no matter how difficult and she followed the lead of Diarmuid.
Several times Aengus Óg the foster father of Diarmuid rescued them when the Fianna were very close to catching them so that never once did Fionn lay eyes on them though he followed them for a year and a half. At last Aengus Óg himself went to Fionn and asked would he not give up the chase and let Diarmuid and Gráinne live in peace. He also went to Cormac Mac Art and asked him the same thing and although neither were all that pleased they agreed as they were getting a bit tired of using their resources in the chase. Cairpre the son of Cormac was not so pleased and became an enemy of the Fianna from that time.
Diarmuid and Gráinne were allowed to settle down on land that belonged to the O'Duibhne's and Fionn even relented enough to give Diarmuid a gift of land as befitted a hero who had served him well in the past. Thus all went well with them for many years until they had four sons and Diarmuid was beginning to lose some of his youthful beauty.
Then one day Gráinne asked him did he not miss his old friends in the Fianna and the hunt and would he not patch things up with Fionn and ask that they go hunting together. Diarmuid thought this over and he realised that he had missed the comradeship of the old days and so he sent word to Fionn to see if he would forget their enmity and go hunting together again.
In the Spring of the year Fionn and a small band of the Fianna came to Diarmuid's house and there was much feasting and talk of old battles. Then Fionn proposed that they hunt on the lands around Ben Bulben which was nearby and they were all glad.
Now there was an animal that Diarmuid was forbidden to hunt as a geis had been laid on him in his youth, and that was the boar, for it was said that by this animal Diarmuid would find his death. Several times in the night he awoke to the baying of a hound and also it seemed the noise of a large pig. He went to get out of his bed to see what was going on but Gráinne restrained him and told him to wait until dawn. When daylight came he could wait no longer. He set out alone with only a single hound for company in search of the noises he had heard the night before. Then he came to a place near the top of Ben Bulben and there was Fionn sitting there awaiting him. The two men regarded each other for a long while.
'Have you come to try and kill me, Fionn Mac Cumhail?' asked Diarmuid suspiciously but Fionn just looked at him askance and said 'One of the hounds of the Fianna escaped in the night and we have been trying to catch him again, but there is a large boar loose on this mountain and it has already killed several of our hounds. You should not be here knowing of the geis which is upon you.
Diarmuid shook his head and said 'I will not run away from any pig. I'll sit here and wait to see what comes.' So Fionn went to find his men while Diarmuid sat in the sunshine on Ben Bulben. Then Diarmuid heard a loud crashing in the bushes below and out of them came the biggest fiercest looking boar he had ever seen. 'Now, I'm done for, I'll see if I can overcome it' and with that he lunged at the boar with his hound at his side. The boar killed the hound in its first onslaught and gave Diarmuid the gash that would cause his death but Diarmuid managed to slay the boar which was none other than his half-brother in the form of a boar. Therefore the old prophecy had come true that if Diarmuid ever hunted a boar he would receive his own death.
Fionn and the men returned later that day to find him almost dead, and Fionn who still harboured a little bit of enmity said 'I'm not sorry to see you like this as you did me wrong.' Diarmuid asked Fionn just to give him some water cupped in his hands, for Fionn had the power of healing. The Fianna agreed and Fionn went to the stream nearby to get some water but as he cupped his hands and took the water he thought of how Diarmuid betrayed him and he grew angry and let the water slip from his hands, then he thought of the Hostel of the Quicken Trees and all the other deeds of bravery that Diarmuid had done and he relented and filled his hands again but by the time he had gotten back to Diarmuid he had already died and it was too late.
Many mourned the death of Diarmuid especially Gráinne who taught her sons to hate Fionn. The years went on by and her anger and hatred of Fionn subsided. So that when Fionn asked her again to be his bride she consented and went to Allmu in Co. Kildare to live out her days with him and she made peace with the rest of the Fianna also.
Note: This is just one of many versions of this tale from Ancient Irish to Scottish sources. It is a story that has gripped the imagination of many writers. It falls under the bardic category of aithead (tales of elopement) one of the twelve categories of tale they would recite.