"'S e do bheatha, a bhalaich."
Translation:You are welcome, boy.
If anyone, like me, was confused how to tell the difference between singular and plural 'balach,' I found a helpful table here: https://en.wikipedia.org/wiki/Scottish_Gaelic_grammar under "example paradigms"
Vocative is the case you use when you speak to someone rather than about someone. And there are various rules about how it is used to alter someone's name or relationship or what they are being referred to. Here balach is masculine, doesn't begin with a vowel sound or l, n or r so gets lenited, with the 'a' put in front and because masculine becomes slenderised (last vowel either has i added after it or sometimes changed to). So balach -> a bhalaich
It's like the difference between 'thu' and 'sibh'. You use do when talking to one of your friends or children or someone you would use 'thu' with. Both 'ur' and 'do' mean 'your' it's just that ur is used when being formal and/or plural and 'do' when you are familiar and on friendly terms with a single person. For the other example you cited they are being polite to their father and using 'ur'. And as you no doubt noticed, do lenites (puts the h in) the following beatha, and ur does not. This is consistent when you use do and ur for 'your'. Don't use them as possessives all over the place though, they tend to be only used with things you cannot really replace (life, country, language, body parts etc.) Literally I suppose this would translate to 'It is your life' but what it means is, you are welcome.