As Finn and Oisin grew older, it became clear to Finn that his first love, Sibh, was lost to him. It was only right and proper that he should have a wife, and one fitting to the new status to which he had elevated himself and his men through their great deeds. It was at a great feast then, that he stood before Cormac and all present and asked for the hand of Cormac's daughter Ailbhe (pronounced like "ale-vee") in marriage. Though it must have galled Cormac to have Finn imply that his social status was equal to Cormac's own, his pragmatic side dictated that he grant the request so as not to alienate and insult the whole of the Fianna gathered there. The two were wed, but their marriage last only a couple of short years until the young Ailbhe died (most sources say in childbirth). Again, Finn asked for another of Cormac's daughters as his wife, and again his request was granted. The daughter's name was Grainne, and what followed became without a doubt the most well-known romance in Irish history.
Grainne (pronounced like "gran-yuh" and literally meaning "ugly one") was, contrary to her name, a beautiful young woman, but Finn was by this time a much older man. They were introduced at a great feast, and Grainne took an immediate dislike to the old warrior, wishing instead to marry someone her own age. Being a high king's daughter, she knew better than to express such distaste publicly, but she determined to find a way out of the situation. There was a young man at the time, a fairly recent initiate into the Fianna, by the name of Diarmaid mac Donn (sometimes also referred to as Diarmaid O'Duibhne). Diarmaid was a devastatingly handsome warrior, and bore a scar on his face which made him even more attractive, and no woman could resist him. In some versions of the story, it is not a scar but a "ball seirce" or "love spot" placed there by some sort of magic. At any rate, Grainne immediately fell deeply in love with him, and asked him to elope with her. Diarmaid, owing his loyalty to Finn, refused, and so Grainne placed a geis (also spelled geas or geasa) on him to take her from Finn. Forced by this sacred magical bond to break his oaths of loyalty to Finn, Diarmaid took Grainne and ran away with her after she put a sleeping-draught in the wine which put the rest of the Fianna to sleep.
When Finn awoke and learned of the treachery, he was livid and declared that he would pursue the pair to their death. Cormac tried to dissuade Finn, but in the end he was forced to give in to him, due to the seriousness of the oaths and laws that the pair had broken. Goll, Oisin and other trusted friends of Finn's also counseled him against this action, but Finn would listen to no man. He assembled a hunting party and pursued Diarmaid and Grainne across the whole of Ireland. Popular sympathy was with the pair, and many were the families who hid them for a night or two. Oisin himself, a close friend of Diarmaid, warned him on multiple occasions of the approach of his pursuers by some secret sign or another. Anghus mac Og, one of the Tuatha de Danaan who was related in some way to Diarmaid, aided their escapes as well. During this time, Diarmuid never laid his hands on Grainne, believing that somehow this was helping him keep some measure of his loyalty to Finn. One day as they crossed a stream, a splash of water touched Grainne's leg and she teased Diarmaid that even the stream was bolder than he. It was then that the two became lovers for the first time. After this, they settled in a hidden forest and Diarmaid built a small shack hidden in the trees for them, as by this time Grainne had become pregnant. Finn discovered them in this place, and some versions of the story say that Anghus helped them escape again, while others claim that Finn spied them in their lovers' embrace, and his heart softened and he left them in peace.
The pair were finally forgiven, and Finn and Cormac gave them some land in Leinster and Connacht where they lived for a time and had several children. One day, Finn and his men were off hunting near their home and invited Diarmaid to join them. Grainne begged him to stay, afraid it was a trap, but Diarmaid was eager to mend old wounds between himself and Finn and joined the hunt. The hunting party was after a great boar, and pursued it to Beann Ghulban. Finn and Diarmaid chased it up a hill, where it turned and charged at Diarmaid, who leapt onto its back. He stabbed it repeatedly with his sword before the beast threw him off and gored him with its tusks, ripping his stomach open. Diarmaid drove his broken blade into the boar's eye, killing it, but he himself was mortally wounded. The other warriors caught up with them then, and Diarmaid asked for Finn to bring him a drink of water from a nearby spring, for it was said that a drink of water from Finn's hands could heal a dying man. Twice, Finn went to get him water. Here, some stories portray Finn as spilling the water on purpose, while others maintain that it simply leaked through his fingers. Whatever the case, Oisin then threatened to kill Finn himself if he did not bring the water a third time, which Finn did, but by this time Diarmaid had died. Oisin and many of the younger warriors blamed Finn for Diarmaid's death, and this was the first major sundering among the Fianna.
The rivalry between Clans Baiscne and Morna flared up again after this, the men of Clan Morna thinking to rise up against Finn while public sentiment was against him. Finn then held a great feast at his home in Almhuin, and to it he invited all the men of both clans, and he arranged for great bards and poets to be present, and great contests to be held, in the hopes that he could repair what damage had been done. But one night as the poets were performing, Goll was showing particular generosity towards them, rewarding them with great quantities of gold and silver. Finn asked Goll how he came by all this wealth, and Goll replied that it was a tribute paid to him yearly by the men of Lochlann (Scandinavia). Finn also exacted tribute from the same source, and there followed some dispute as to whether Goll's tribute lessened Finn's. Goll explained that he earned the tribute when he wrought much havoc in Lochlann after being exiled there by Finn's father Cumhail when he was Rigfennid. He went on to say that he paid Cumhail back handsomely for this insult when he dealt him his deathblow at the battle of Cnucha. At this, Finn and the members of Clan Baiscne went into a rage, and there was a great deal of bloodshed. The men of Clan Morna refused then to acknowledge Finn's leadership any longer, and they left, vowing a blood feud on Clan Baiscne.
Clan Morna made several raids on Clan Baiscne, who then counterattacked and pushed Clan Morna back to the river Shannon. Clan Morna stopped there, determine to hold the fords of the Shannon and prevent Clan Baiscne's further progress. Late one night, Finn heard the snoring of Goll and crossed the river quietly to where he was sleeping. Standing over Goll with his sword drawn, he generously allowed Goll to gather up his weapons and ready himself. Just then, a large number of the sons of Morna arrived, and Goll repaid Finn's generosity by allowing him to return to his men unscathed. The next morning, Clan Morna was forced to abandon their stand. Clan Baiscne had the upper hand in regard to numbers, and they pursued Clan Morna ruthlessly. In the ensuing skirmishes, one of Finn's sons was slain by Goll's hand, and Finn went into a rage and the sons of Morna were routed. Goll himself took refuge in a nearby cave, where Oisin's son Oscar approached him to try and effect peace. But Goll, thinking he came as an enemy, cast a spear at him and broke his shield. When Finn heard this, he ordered his men to surround the cave that Goll might not escape. Goll's wife arrived shortly before, and a long poem describes a conversation they had, in which is described Goll's storied career, his prowess as a warrior and worth as a husband and a man of honor. Accounts vary here, some saying that Finn's men starved Goll to death, others that the two warriors met and solemnly reconciled before engaging in single combat, in which Finn slew Goll. Either way, it is here that the mighty Goll mac Morna met his end.
When Finn returned to Tara, he found a new high king there. While he was away fighting Clan Morna, a raiding party from another clan had attacked Tara. Cormac, also long in his years, determined that he would lead his men in battle. The battle was won, but Cormac suffered the loss of an eye. Irish law stated that no man with a blemish could be king, and Cormac was thus forced to yield the throne to his son Cairbre. Cairbre had no love for Clan Baiscne, due to their earlier abduction of him to shame his father, and in light of their recent campaign against Clan Morna, he declared them all outlaws and enemies of Tara. And it was then that the two sides went and gathered their respective armies, and met to do battle at the hill of Gabhra. Many men on both sides died that day, but it was Oisin's son Oscar who cast his spear and slew Cairbre, after having received his own death-wound from that same king. Few of the Heroes of the Fianna survived that battle aside from Finn himself, his son Oisin, and Caoilte mac Ronan the great runner.
Now Tara without a king, and the Fianna both divided and decimated, it was a sad end facing Finn mac Cumhail. He spent the rest of his days wandering and hunting with his hounds, until one day he again came upon a young fawn who was familiar to his old eyes. Bran immediately leapt up to give chase and Finn struck her, fearful that she would harm the fawn whose name he dared not hope to speak. Bran, by this time old beyond the age of any hound, yelped one last time and fell dead upon the spot, and at this Finn wept bitterly.
Little is written or spoken of Finn's death. Some say he went with the fawn, who was indeed his beloved Sibh, into the land of the Tuatha de Danaan, named Tir na nOg, the land of eternal youth, and there lived with her forever. Others say that it was once prophesied to Finn that if ever he drank from a horn he would die, and so he always drank from goblets. One day he took a drink from a spring at a place called Adharca Iuchbha (literally, "the horns of Iuchbha") and chewing his thumb after, he knew his death was near. He was later surrounded by a group of Connacht mercenaries, the Ui Neill who were always enemies of Finn's, and killed by them. Other stories insist that Finn is still alive, or is sleeping in a great cavern somewhere in Ireland.
Oisin is said to have gone with Caoilte to Tir na nOg, where he married a beautiful Sidhe princess and lived for hundreds of years until he grew homesick one day. The princess sent them to Ireland on a magical horse, but warned them not to set foot on Irish soil or they would never return. The horse was frightened by a snake and threw Oisin and Caoilte, who turned into old, gray men when they touched the turf. They were then discovered by St. Patrick, to whom they recounted all the great tales of Finn and the Fianna, and then were baptized Christians by St. Patrick before passing away. The Saint then rid Ireland of snakes in the name of the two heroes. Clearly a bald-faced post-Christian invention (Ireland was well-known in ancient times for its lack of snakes, as mentioned by the Greek writer Solinus over a hundred years before Patrick's birth), it is nonetheless part of the legends surrounding Finn mac Cumhail.
This brings us to the end of this condensed version of the storied life of Finn mac Cumhail. For some general commentary, and suggestions for further reading, please refer to the final section The Life of Finn mac Cumhail 5: Epilogue or go back and read The Life of Finn mac Cumhail 3: Finn's Adulthood if you haven't yet.