If you want some inspiration for playing a Fianna or running a Fianna campaign, you could do worse than looking up these sites:
Fianna Tribe Homepage: Don't be intimidated by all the Gaelic. It'll pass.
Fianna Organization. I'm not sure what to make of these people, but they certainly take reliving the old ways seriously. Their links page is especially useful (scroll down to the bottom of the page).
Of course, check out some of the sites listed on in the British Isles page.
CelticTaps: a database of Irish, Scottish and Welsh pubs.