"Good afternoon, Fergus."
Translation:Feasgar math, Fhearghais.
You use a before the name of someone you are addressing unless the name begins with a vowel or an F followed by a vowel.
In the vocative case you lenite the initial consonant where possible (and slenderise the final consonant with masculine names). As F lenites to Fh, and as Fh is almost always silent, this means that a name beginning with F followed by a vowel in effect begins with that vowel in speech when the F is lenited to Fh. So, as Fionnlagh lenited and slenderised to Fhionnlaigh effectively begins with a vowel, you don't need to use the a particle before the name in the vocative case.
With names beginning with F followed by a consonant, e.g. Flòraidh, the name lenited to Fhlòraidh effectively begins with L in speech, so you still need to use the a particle - a Fhlòraidh.