As a male Irish name it doesn't sound like the most masculine name but it has great history and meaning behind it - I wouldn't use it but a traditional middle name may make it sound better - E.g, Fionn James, Fionn Michael etc.
Fionn is a perfect masculine form for Fiona. I think people assume lots of masculine forms of feminine names are still feminine but if you want to use Fionn for a girl, I suggest following it up with a feminine middle name because Fionn is traditionally masculine. As for Fiona, the middle name wouldn’t matter,
― Anonymous User 1/9/2019
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Unless, of course, you didn't want your new son to have a spelling the same as some actor who overdosed whose TV character was spelled "Finn". Ugh.