"Hello and thank you, boy."
Translation:Halò agus tapadh leat a bhalaich.
The vocative case; used when addressing someone or something.
You stick an "a" in front (unless the name starts with a vowel), lenite if possible, and if masculine you slenderise if possible (make last vowel an "i").
Although "boy" is literally what's said, the understood meaning is more like "mate/pal".
Leibh is more formal/respectful and/or plural ... like "vous" and "tu" in French. Gàidhlig even has the equivalent of "vouvoyer" == «sibhsigeadh» ("to address someone with sibh") which means to use sibh and related forms (leibh, agaibh, oirbh, etc). Not sure if there's a "tutoyer" equivalent.