Fiannas role in a Kelt force... they are a mid-cost troop and are useful because they provide high INI and DEF (compared to other Kelts). They work best supporting low INI troops like Giants or Minotaurs. Fianna Warstaff is also currently the strongest (i.e. Veteran) for Sessairs.
Not recommended as bait because there are cheaper Kelt Warriors with the same RES for that... Actually almost all the standard troops have a RES of 3, so best to send the most expendable
I wouldn't really recommend Koren, as you won't fit him in the same army as Virae (both cost alot of AP). IMO from a pure gaming point of view, Giants and Sons of Ogmios are best parnerships for Fianna.
Spasm Warriors can be hit or miss... Both rules for spasming have their drawbacks and they ars a little less effective compared to in C2... Definitely alot of fun though.
My 2 cents,