The Fianna were Irish warriors who served the High King of Ireland. Their last leader was Finn mac Cumhail (or Fionn mac Cumhail, anglicized to Finn McCool). The son of Finn mac Cumhail was the poet Oisín. In the 1760s James Macpherson published a series of poems, which he claimed to have translated from old Scottish Gaelic language poems, mostly written by Oisín. In real he had used the celtic legends and written the whole book himself, based on their themes. But the "Ossian" became famous because of the romantic era, it was read and spread by Johann Wolfgang von Goethe and Felix Mendelssohn.
There are numerous variations about Finn's dead, but the most popular tells that he is not dead at all. He sleeps in a cave below Dublin, to awake and defend the city in the hour of her greatest need. This part resembles very much the story of Emperor Barbarossa.
Another cave related story about Finn is the creation of Fingal's Cave and Giant's Causeway. The legend from the Ossian tells that Fionn mac Cumhail created a sort of land bridge or series of stepping stones between Scotland and Ireland, to be able to travel without getting wet feet. Other tales tell he built the Giants' Causeway as a footpath for his wife to reach her throne on Staffa. Obviously such legendary action, like building land bridges, requires some strength. And so some legends descibe Finn as a Giant.
Fingal's Cave was named by Mendelssohn, after Finn, and so the following version of the legend seems to be a mixture of various elements and both protagonists are actually the same. We suppose there are today dozens of different versions told.
Fionn mac Cumhail lived on a headland in county Antrim. One day a Scottish Giant named Fingal began to shout insults from across the channel, until Finn became angry. Finn took a clod of earth and threw it at the giant, but it landed in the sea. Fingal answered with a huge rock and shouted that Finn was lucky that he wasn't a strong swimmer or he would have made sure he could never fight again. So Finn started to throw huge clumps of earth from the shore, throwing them so as to make a pathway for the Scottish giant to come and face him.
However by the time he finished making the crossing he had not slept for a week and so he did not want to fight him immediately. With the help of his wife Finn diguised himself as a baby in a cot. His wife told the Giant that Finn was away but showed him his son sleeping in the cradle. The Scottish giant became scared what size the father would be. He returned to Scotland, but in his haste to escape he teared the passage up. He is said to have fled to a cave on Staffa which is to this day named 'Fingal's Cave'.