The modern Fianna are the Celtic Reconstructionists that follow the path of the warrior but have no calling to be part of a "tribe" or grove. We live and worship separate from the grove and though we may attend the rituals of a tribe we don't take part. The modern Fianna will be the security at the events you attend and when not working on our martial skills or other studies we will work in organizations that will ensure religious rights.
You do not have to be of any particular lineage or race to join in the Fianna. You just have to be willing to meet the demands that the gods will place on you as you travel this path. It is lonely and oft times it is rough as we tend to be the ones who find ourselves in the midsts of "wars" among ourselves and even groups we do not belong nor truly care to belong. We tend to try and correct "wrongs" or act as intermediaries among fighting parties, and sometimes, we are the fighting party.
As I said in the Welcome, these are just the basic ideas, however I would like you to read an article, or rather this portion of an article. The entire piece is called The Ethical Paradigm of Druidism and I do recommend reading it, but here is the Moral Principle of the Fianna.