"Good morning, Fergus."
Translation:Madainn mhath, Fhearghais.
I was wondering this too, so I dug around the tips and notes. basically it has to do with the F being followed by a consonant or vowel (before lenition).
for feminine nouns, the vocative case for names that begin with F followed by a consonant, we add the vocative particle a.
eg. Flòraidh > Halò, a Fhlòraidh
for masculine nouns, the vocative case for names that begin with F followed by a consonant, we add the vocative particle a.
eg. Frìseal > Halò, a Fhrìseil
but for names that begin with F followed by a vowel, we ommit the vocative particle a.
eg. Fionnlagh > Halò, Fhionnlaigh
eg. Fearghas > Halò, Fhearghais
(this is just me copying the info from the tips. it didn’t see anything about a feminine name starting with F being followed by a vowel… I may have simply not come across that yet)
It's part of the same process. The vocative case causes lenition with certain letters (like C and F), and it can cause a slenderisation of the final consonant - that's the "i" that is introduced before the"s".
You'll see more examples in the tips/notes for the course here under "the vocative case": https://www.duome.eu/tips/en/gd