I think that it is an utterly enchanting name, personally. However, it is much too... pretentious? To bestow onto a child outside of Ireland. As always, I suppose of you have a large amount of close heritage stemming from Ireland, and possess a surname to pair with, it has the potential to be successful.
If you have no Irish blood, the name may as well be "kre8tiv." A Fionnuala Mac Pharlain has a chance-- not Fionnuala Adamsen. It is amazing how a simple last name holds the power to either suggest, A). Being the definition of subtle beauty and of maturity, to B). Appearing to be a "kre8tiv," pretentious, pompous atrocity. Future employers will be wary of any name that seems unprofessional-- a name much like Fionnuala if you are not cautious.
Albeit I wouldn't name a daughter this, in my opinion anyone would be SOOO lucky to have this name. I mean, why the heck ISN'T it popular? It has everything... a beautiful sound, rarity... it doesn't sound like a trendy or teeny-bopper name at all, your daughter won't need to worry about being "Fionnuala C." or "Fionnuala W." all her life, it ages marvelously, and "Nuala" sounds like a beauteous and melodic nickname on its own. I wish it were MY name.