"Very good, Niall."
Translation:Glè mhath, a Nèill.
Yes :( It's a lot easier to add a comma in the English than it is in the Gaelic. To change the Gaelic sentence, we'd have to delete it and re-add it, losing the audio recording and the alternate translations in the process. One day they will all get fixed, but I'm not willing to do it until I know I can have the audio rerecorded straight away. I am also a bit of a pedant, it bugs me too :)
"a Niall" isnt the vocative case. The speaker is addressing Niall, so it needs to be the vocative "a Nèill". General rule when you're addressing someone is "a"+lenition(+slenderisation if it's a male name). To slenderise, the last vowel gets a slender vowel (i or e), which affects the sound of the last consonant. So Màiri:a Mhàiri, Seumas:a Sheumais, Niall:a Nèill. If it helps to remember, the name Hamish (=how the lenited and slenderised word Sheumais is pronounced) came into existence in the first place because non-Gaelic speakers overheard Gaels trying to get the attention of Seumas. :-) Exactly how to slenderise individual words comes with practice. :-D
The Tips for this lesson (Names 2) cover male names in the 'vocative' case:
...including change of 'Niall' (nominative) to 'a Nèill' (vocative).
Because the sound changes are there, and are part of the grammar of the language. :-) The pronunciations are different, if only subtly in this case - "Niall" is somewhat similar to "knee-al", where "Neill" is a bit more like "nail" (of course it varies by dialect and accent, and these are at best only very approximate transcriptions!). In Duolingo, the older lady's pronunciation of the two seems to me very distinct.
Other names have much less subtly different vocative cases, e.g., Seumas/Sheumais.