OP this thread is the perfect example of why this is probably a hard one to carry off.
Practically nobody is at all familiar with the Irish boy's name Fionn, pronounced (roughly sort of ) like Finn.
There is enough familiarity with the Welsh girl's name Ffion for a lot of people to immediately start blethering about Ffion and have you spelled that right? and oh I thought it was a girl's name? and oh hello Fee-on I expectecd you to be a girl! etc. etc.
Lovely name, up to you if it's worth the hassle, personally I think yes, it's great.