"Milk! Thank you, father!"
Translation:Bainne! Tapadh leibh athair!
From what I have read (hopefully accurate), the vocative case is formed by a) leniting the beginning of the word (m --> mh, s --> sh, etc) b) slenderising the end of the word if it ends with a broad consonant (bràthar --> bhràthair) and c) putting 'a' in front of the word, unless it begins with a vowel or f, since f becomes fh which is silent, (I.e. fear becomes fhear and not a fhear).
Hope this helps anyone wondering about this.