The game authors designed the Fianna as a passionate tribe of Garou, descended from Celtic cultures. Widely known for their storytellers, bards, musicians, poets and other artistic types, the Fianna are a tribe whose love for parties, music and company is coupled with a strong sense of honor, potent ferocity, and strength in combat. The Fianna claim that the first Galliard was a Fianna and they're known for their ties to the fairy-folk (see Changeling: The Dreaming).
Every Fianna has a story to tell, their bold tales are only matched by the Silent Striders. Fianna lust for a good fight is matched by the Black Furies, whom they tend to hit on at grand moots, and Get of Fenris. Even though they share a common ancestry with the Gets, they are on bitter grounds with each other due to several conflicts the two tribes have had. The Fianna have warm relationships with the Children Of Gaia and Bone Gnawers and (mostly) obey the reign of the Silver Fangs.
While the tribe didn't originate on the British Isles, they now consider it their homeland (especially Ireland). They forced the White Howlers, who lived there before, to move into Scotland; while they overtook England, Wales and Ireland. With the first kings and queens, the Silver Fangs came to the isles to claim them their kin. Later the Get of Fenris invaded along with their Viking kinfolk (source of their bitter relationship). The isles were a relatively calm place otherwise, since they were well protected. But when the Wyrm managed to corrupt kinfolk among the local Garou and turned the White Howlers into the Black Spiral Dancers, the relative peace suddenly ended. Today Fianna still fight anyone with passion who bears wyrmish taint. Like their kinfolk, the Fianna of England and Ireland developed a grudge against each other. With the rise of the British empire, Fianna settled in America and in lesser numbers in Australia. They never settled in the British colonies in Asia, where the shapeshifter population is governed by the Beast Courts of the Emerald Mother.