By far the most passionate and tragic story of the saga is that of Diarmaid and Grainne, to whom Fionn was betrothed.  Grainne put geasa upon Diarmaid to elope with her, and these he could not break.  They fled, and for many days were pursued by Fionn, who at last overtook them, but was forced by the Fians to pardon the beloved hero.  Meanwhile Fionn waited for his revenge.  Knowing that it was one of Diarmaid’s geasa never to hunt a wild boar, he invited him to the chase of the boar of Gulban.  Diarmaid slew it, and Fionn then bade him measure its length with his foot.  A bristle pierced his heel, and he fell down in agony, beseeching Fionn to bring him water in his hand, for if he did this he would heal him.  In spite of repeated appeals, Fionn, after bringing the water, let it drip from his hands.  Diarmaid’s brave soul passed away, and on Fionn’s character this dire blot was fixed for ever.[512]

Other tales relate how several of the Fians were spirited away to the Land beyond the Seas, how they were rescued, how Diarmaid went to Land under Waves, and how Fionn and his men were entrapped in a Fairy Palace.  Of greater importance are those which tell the end of the Fian band.  This, according to the annalists, was the result of their exactions and demands.  Fionn was told by his wife, a wise woman, never to drink out of a horn, but coming one day thirsty to a well, he forgot this tabu, and so brought the end near.  He encountered the sons of Uirgrenn, whom he had slain, and in the fight with them he fell.[513] Soon after were fought several battles, culminating in that of Gabhra in which all but a few Fians perished.  Among the survivors were Oisin and Caoilte, who lingered on until the coming of S. Patrick.  Caoilte remained on earth, but Oisin, whose mother was of the sid folk, went to fairyland for a time, ultimately returning and joining S. Patrick’s company.[514] But a different version is given in the eighteenth century poem of Michael Comyn, undoubtedly based on popular tales.  Oisin met the Queen of Tir na n-Og and went with her to fairyland, where time passed as a dream until one day he stood on a stone against which she had warned him.  He saw his native land and was filled with home-sickness.  The queen tried to dissuade him, but in vain.  Then she gave him a horse, warning him not to set foot on Irish soil.  He came to Ireland; and found it all changed.  Some puny people were trying in vain to raise a great stone, and begged the huge stranger to help them.  He sprang from his horse and flung the stone from its resting-place.  But when he turned, his horse was gone, and he had become a decrepit old man.  Soon after he met S. Patrick and related the tale to him.