The O’Bannon is a troubled man, and he needs his knights to help him look after the people and crown of Inismore. When he rules, they stand by his side. When he is gone they watch for his return. The Fianna are the eyes and ears of the O’Bannon, and the heart of Inismore.
When the O’Bannon came into power many years ago he gathered his best knights around him and took them into a dark room deep in his castle and locked himself in with them.
“Whom do you serve?” he shouted. One said, “You, my lord,” and O’Bannon cut him down where he stood. “Idiot,” he screamed as he tore at his hair, “You serve a madman, what good is that? Who will see to my people when I am gone?”
Another said, “Then we serve the people, your Majesty,” but the O’Bannon drove his blade into the
man’s heart and watched him fall dead to the floor. “Imbecile,” cried the King smashing his head against a wall until blood ran into his eyes. “The people will die and move on. They will make themselves King after me. They will hurt and kill each other and bring ruin when I am gone.”
Finally one knight stepped forward and said, “I serve Inismore and that will be an end to it, great lord.” The O’Bannon came up to that knight, so close he almost touched, and fixed him with a shattered stare from his broken eyes. “Then, sir, you serve a great mistress,” said the O’Bannon. “You will sing for me and watch when I am gone. Now tell me, good sir knight, who here do you trust?”
A little unsure, the knight pointed out those among the other guards whom he felt he could trust and who stood as his friends. Smiling, the O’Bannon slaughtered the rest, and named the survivors “The Fianna” or Soldiers.
The Fianna serve the interest of Inismore above all else. This is a double-edged sword. On the one hand, the Inish people know they exist to protect them at all costs and act in fairness and justice. That said, they have a "the ends justify the means" philosophy. If burning a village and all its people to the ground - Inish or otherwise - will serve the greater good, they will do so without mercy or remorse.
The Fianna disapear with O'Bannon when he goes on his walks. No one knows if they merely disband or disapear in the same way O'Bannon does.
They have three duties as an organization: First is to protect Inismore at all costs. Second is to protect Old Molly, the Fal Stone gifted to them by the Sidhe. Finally, Third they follow the will of O'Bannon. That said, they have been known to break this third vow at times when they think his judgement is flawed and are perhaps the only group the O'Bannon doesn't destroy for not listening to him.