In reading about the Fianna under the command of Fion Mac Cumhel we find a passage regarding the requirements of the candidate. They must be versed in "twelve books of poesy", run at high speeds in wilderness without leaving tracks or mussing his hair, defend against nine thrown spears with only a shield and hazel stick, if none of the tests are passed they were not permitted into the Fianna (Rees 63). With the exception of the memorization of verse, these requirements are fairly impossible to meet, even by people as fit as the warriors of the Iron Age. Yet if we look these requirements we can find traces of what the requirements possibly were in reality as well as a glimpse at the final initiation of the Fianna.
The Fianna, even under Fion, were outcastes(Nicholson). They had no claim to land, money or title, and should anything happen to them, the family of the dead Fennid could not collect an "honor price". This was done, prior to being accepted, in the form of having the candidate proclaim that he is breaking his familial ties and relinquishing his clann ties (Rees 63). It has been suggested that the Fianna were the youngest children of families, since they would not inherit much, if anything from their parents (Prydwyn). Being outcaste they were denied access to the rituals and lore of the tribes to which they once belonged. As a result it seems necessary that each Fennid would have to know aspects of each of the three functions of society
Each Fennid was expected to be able to recite twelve books of poetry. We do not know if this is a literal act or a metaphorical one but we can examine why this was required. The lore-keepers of the society are part of the first function, they were the Druids and Fili that kept the tales and myths alive and made them available to the people. As has been pointed out, "...poetry and stories weren't recited for mere entertainment, but for ritual occasions: they were a bardic sacrament."(Kondratiev) Thus the importance for each Fennid to be able to recite poetry is clear.
The second requirement is less obviously the third function aspect of the Fianna. The requirement to be able to be hunted without misplacing a braid, or missing a step while removing a thorn or leaping about without breaking a twig, all fits into the huntsman of the third function. The purpose of this test could be to see if the hunted would make a good hunter by turning him into the prey.
Finally, the initiation itself is also the test of warriorship, or the second function requirement. The initiation of the candidate into the Fianna, as it is written, has the candidate placed in a hole up to his waste, given a shield and hazel stick and is made defend himself against nine warriors who throw spears at him, if he is wounded he is not accepted into the Fianna (Rolleston 264). Taken as a literal event alone we know that unless all nine warriors are casting the spears from the same general location we know that the candidate will undoubtedly get wounded.
Take the same event and look at it metaphorically and we have the typical death and rebirth rite found in Indo-European cultures. The candidate is placed into a womb-like area, killed and then removed from the womb again as if being born. In this case the womb is the hole to the waste and the ritual killing is done by the nine warriors with spears.
As for the rite itself, we do not know how the rite was actually implemented but at this time but according to one source all nine warriors stood together and cast the spears at once (Rees 63). While this may seem to fail as an actual test, having had people throw items at me I can attest to the fact that being able to block nine simultaneous attacks, even from one direction, takes a bit of skill. This would also instill the fear of death or shock, which is required for an initiation rite, without undue risk of actually killing the candidate (Campbell 23). The candidate is then born into the Fianna thus symbolically breaking all ties to family and clan, which is required of all the Fianna.