Many would agree that of all the warriors who served Fionn Mac Cumhail in the Fianna, Diarmuid O'Duibne was the greatest; no task was too great that he would not try it. He was extremely handsome and attractive to women which caused sorrow to the Fianna in time to come. As a boy he went to be trained with the warrior woman Mongfinn, but when he was old enough he left and went to Allmu of the White Walls to take service with Fionn.
On the way himself and his companions came to a ford in the river and by its banks was an old woman who begged them to carry her across. All of the boys refused fearing to dirty their clothes with the mud of the river. Diarmuid took pity on the old woman and waded across the river with her in his arms. As he reached the other side his companions having gone on without him, he found the old woman had turned into a shining figure that he recognized was one of the Lordly Ones from Tír-na-nÓg.
She smiled at him and said, 'Diarmuid O'Duibne, you are a good son and shall be rewarded for your act of kindness to an old woman, I promise that no woman will be able to refuse you or resist your gaze. Also you have not seen the last of this place for here shall be another feat attempted that shall be remembered.' And with that she vanished.
Then Diarmuid travelled on to Allmu of the White Walls to where Fionn lived and told him the story of the old woman. Fionn put his thumb in his mouth and divined that the old woman was none other than the Morrigan goddess of War, Life and Death. He was also troubled about the gift she had given Diarmuid as he felt it could cause him heartbreak.
Many years later Fionn fought a battle against the King of Lochlan, and slew him and all his sons except the youngest Midac whom he took into his own house and afterwards gave lands on the coast. In all that time Midac was never thankful towards Fionn, rather he secretly hated him for murdering his kin and so he plotted against Fionn.
One day while the Fianna were out hunting, to the west of Allmu, Fionn and a few men got separated from the rest of the party who were following the tracks of a boar towards Cnoc Fianna. As they rode along they came upon Midac who greeted them with a smile (although there was some of the Fianna who knew his true colours) and invited them along to have something to drink at the Hostel of the Quicken Trees which was nearby.
Fionn agreed to this and ordered that some of the group namely Oisín and Diarmuid, Fodla and Caoilte Mac Ronan, and his youngest son Fiachna with his foster brother Innsa, should remain behind until the remainder of the hunt caught up with them.
Then Fionn and Goll Mac Morna, and Conan Maol and the rest, rode after Midac to the Hostel of the Quicken Trees.
It was a fair and beautiful building, with bright intricate carvings on the wood of its uprights and a fresh thatch that shone in the sun like gold, and all around it grew quicken trees with berries full and red on them.
Fionn and his men followed Midac within and were amazed at the richness they beheld: fine hangings on every wall, soft couches to rest on, and a bright roaring fire in the hearth to warm them. 'Certainly you have done well for yourself' said Fionn to Midac but when he looked around his host was nowhere to be seen. 'This is very strange and no servants to be seen either' he said.
'There is something stranger than that' said Goll, 'A moment ago the walls were hung with fine tapestries and now they seem but rough planks through which the wind blows.'
'There is something else strange' said Fiachna, 'When we came in there were seven doors but now there is only one, and it closed tight.'
'What is more,' added Conan 'these soft couches we were sitting on seem to have become as hard as the earth...perhaps because they are!'
Seeing treachery and magic thus revealed, the Fianna made to escape from the hall, only to find that they could not move, but were anchored fast to the floor as though with bands of iron. Then Fionn put his thumb of wisdom in his mouth and groaned aloud: 'Now is the treachery of Midac Mac Lochlan revealed,' he said 'for I see a great host coming against us, led by Sinsar of the Battles, and his son Borba the Haughty, and coming with them are the three sons of the King of Torrent. It is this last one whose spells hold us here and only the scattering of their blood on the floor will set us free but there is little chance of that.'
Then the men of the Fianna sent up a war-cry, the Diord Fionn and so loud was their cry that Fiachna and Innsa heard it and hurried down to the Hostel of the Quicken Trees to discover what was wrong. When they heard the story they felt their battle-rage come upon them and they said they would defend the ones inside no matter what the price.
So they went to a ford nearby that the enemy host must cross in order to gain access to the hostel.
Meanwhile their enemies had set up camp a few miles distant. It occurred to one of Midac's chieftains that if he went with his own men and slew Fionn and brought his head back to his master great fortune would be his. So he set off and came to the ford where Fiachna and Innsa waited, there followed a bloody battle in which Innsa fell to Midac's chieftain and Fiachna in turn slew him and so many of his men that only a few returned to Midac.
Fiachna buried Innsa in a shallow grave and bore the news of his death and the slaughter of their enemies to Fionn, who wept for the death of his foster-son. They were still held fast and Fiachna refused to abandon them and go get the rest of the warriors.
Meanwhile another chieftain of Midac's named Ciaran, fell to wondering why his brother had not returned and set forth in search of them. When he came to the ford he found it choked with the bodies of the slain, and Fiachna waiting to meet him.
Then there followed a tremendous battle, Fiachna managed to hold the ford against dozens of attackers and at the end only one man escaped to take the news to Midac. Who was so enraged that he went himself to the ford where he found Fiachna leaning on his sword and bleeding from many wounds. When Midac saw all the bodies of his men where they formed a wall of dead in the stream, he flung himself forward and engaged the young hero in single combat.
Meanwhile Oisín, Fodla and Diarmuid had been waiting to hear word back from Fiachna and Innsa. When they none came Diarmuid and Fodla went down to the hostel to check out what had happened while Oisín went back to see if he could find the rest of the Fianna. When Diarmuid and Fodla neared the place they heard the sounds of battle they started running and arrived just in time to see Fiachna hard-pressed against the onslaught of Midac and just about to give way, without pausing in his stride Diarmuid threw his spear which took Midac in the chest and laid him on his back but it was too late for Fiachna who died of his injuries.
Then as Fodla fought off the rest of Midac's men, Diarmuid cut off his head and went with it to the Hostel of the Quicken Trees. He got the story of all that had happened from Fionn and groaned when he heard who was involved but said himself and Fodla would hold the fort against all comers until Oisín arrived with the rest of the Fianna.
Diarmuid went back to the ford to help Fodla and discovered that Fodla had managed to drive off the few remaining men of Midac's war band. He was so exhausted that he fell asleep as soon as he saw Diarmuid and Diarmuid covered him with a cloak.
Then the three sons of the King of Torrent came, and there for many hours Diarmuid held them at bay single-handed not liking to wake Fodla. The noise of the battle eventually woke him and he was most annoyed for having missed the action. The two of them together managed to drive back the enemy with dreadful slaughter. In the end Diarmuid slew all three of the King's sons and took their heads. Then while Fodla chased off the rest, he took their heads to the Hostel of the Quicken Trees and sprinkled the blood on the doorstep and on the earth within. Now Fionn and his men were able to move but they were still in a very weakened state and Fionn told Diarmuid that they would not regain their strength until the next day.
So once more Diarmuid returned to the Ford of the Quicken Trees and there he and Fodla awaited the last of their enemies Sinsar of the Battles and his son Borba with all their men. This was the fourth combat and they had to fight until after sunset. Diarmuid advised Fodla to save his strength until the rest of the Fianna would arrive with Oisín. In the early light of dawn Sinsar and Borba and also the King of the World with all their men arrived and things might not have gone too well for Diarmuid and Fodla but with the first ray of the sun came Fionn and Goll and the others who had regained their strength, chanting their battle cry they joined in the fray. Things might still have turned out bad but before the morning was over Oisín arrived with the rest of the Fianna. With all his men at his side Fionn rose up in his battle fury and killed so many that very few lived to tell the tale of their defeat.
Among the slain were both Sinsar and Borba the sons of the King of the World. After that it was a long time before anyone challenged the Fianna. It was only afterwards as he rode from the Hostel of the Quicken Trees which they left in flames that Diarmuid realised that this was the same ford that he had carried the Cailleach (means hag in Irish) Morrigan over many years ago.