There is, it’s just the same as nominative. For many feminine names the vocative has the same form as nominative, just gets the leniting particle a before (eg. nom. Mòrag vs voc. a Mhòrag). But vowels can’t be lenited and the particle a is usually not written before vowels, so feminine names starting with a vowel remain unchanged: nom. Anna, voc. Anna, nom. Iseabail, voc. Iseabail (or sometimes a Anna, a Iseabail but pronounced the same as just Anna, Iseabail).