It makes sense when you hear the pronunciation. The Fh is silent so the name effectively starts with a vowel.
Why is there an a before Fhriseil, but not before Fhearghais.
Names that begin with F are a unique case.
If it's F + a consonant, it keeps the "a"
If it's F + a vowel, it doesn't get the "a"
Both of them lenite (the "h" after the F). Masculine names slenderise (the "i" before the final consonant).
Oh, of course, as if the silent fh weren't there. Thank you very much.
Why not Fearghas?
If you were talking about Fearghas then that would be correct. Since you're talking to him, the case changes (similar to how caraid becomes a' charaid when you call someone friend).
I was going to type in tìoraidh but when hovered over 'mar sin leat', did not see that option. Would it be wrong?