It concerns the deeds of the warrior-hero Fionn mac Cumhaill, his fianna, and, eventually, the gods and goddesses of Irish myth as they defend Ireland against a foreign invasion led by the world-king Dáire Donn.

[1] Dáire (or Dáiri) Donn, called "king of the great world" and ostensibly the most powerful ruler in Europe, intends to invade Ireland.

First of all, Dáire seeks retribution for the fact that Finn has eloped with the wife and daughter of Bolcán (Vulcan), King of France, when in the mercenary service of the latter.

Finn's son Oisín faces Bolcán in combat, who much like Suibne Geilt, goes insane and flies off, ultimately landing at Glenn Bolcáin.

The event is first referred to in a love-story of the Acallam na Senórach, in which Finn sets out to meet the enemy in battle, but interrupts the journey to act as a match-maker for Cáel and Créde.

He noted several burial carns near Cantraw, presumed evidence of a great slaughter, along with local claims of finds of many bones and skulls at the east end of the strand.