I like
Fenella; it seems pleasantly restrained and not too uch of a conversation-stopper - unlike Fionnula, for instance. Between
Fenella and
Fiona, I'd go for
Fenella.
I can't make sense of popularities in that way. I think there's a core of names that have always been used, and they seem to percolate up and down the charts - parents avoid the names of their own parents but are more likely to enjoy their grandparents' names, for instance. And then around that core there are various names that survive for a while and then sink.
It can't only be sounds, otherwise
Alan and
Stephen would be up there with all the -an and -en boys' names. So it must be generational, in that case anyway.
Gosh - and you were wondering if you made sense! You and me, pal!