Thank you both. This pretty much explains everything I was looking for, and I appreciate the help. The bible has simply retained the older, i.e. traditional versions of these words–and after all, that’s what bibles do, all over the world! I now feel comfortable just sticking with naofar and féichiúna as I learn the prayer, as they sound better than naomhaítear and féichiúnaithe in this context.
Go raibh maith agaibh faoin gcabhair, a chairde!