Translation:Sgoinneil, a Nèill!
Niall is the nominative-accusative of the name, the basic form used as the subject or the object of a sentence, so eg. tha Niall ann Niall is there, chì mi Niall I see Niall, etc.
But the vocative is a Nèill, so to address Niall you say a Nèill eg. in dè as toil leat, a Nèill? what do you want, Niall?
Masculine nouns often form plurals by changing their last consonant to a slender one (marked by i before the consonant, like in Nèill) and vowel change to more fronted one (eg. a to è like here) – a process called attenuation or slenderization, see Masculine nouns on Akerbeltz wiki.
And compare eg. each a horse, (a) eich (oh) horse!; balach a boy, a bhalaich (oh) boy!.