Hey there I was wondering if anyone could double check some grammar for me, as I'm not huge on my grammar in English let alone Gaeilge.

I take the term 'Mac clann xyz' to mean 'son of clan xyz' or 'son of the clan xyz', is that right? Or am I missing something important out? Does it just sound weird? Tá!

Sounds a bit off to me. Since the literal meaning of clann is "offspring" or "descendants", I would expect a partitive expression with de rather than a noun in the genitive. Cf. "fear de chlann Iosrael" (An Bíobla Naofa).

Hmm thanks I was worried about that, does anyone know a Irish term for a broad family collective? Similar to how we would think of a clan in English?

Actually, would something like 'Mac muintir xyz' work better?

Depending on the context, muintir might work. But I would think you'd run into the same difficulty.

Maybe I'm a bit confused as to what you're trying to say. There's a difference between being a member of a group and one of its possessions.

Moreover, my impression is that in Irish, the possessive tends to imply exclusivity. Thus mo chéile vs cara liom.

I think I'm confusing my self a bit , I was reading about Ogham inscriptions no too long a go and I came cross the formula inscription X MAQI MUCOI Y (X son of the tribe Y), what I'm trying to do is come up with some kind of modern equivalent but instead of 'tribe' some slightly tighter family group?

Isn't this essentially how Ó functions? Cf. Mac Uí Mhathúna "Mr McMahon".

I think you're right, I'm just over complicating things

Though wouldn't 'Mac de fine xyz' work as well? If not a little over complicated?

You really need to ask a fluent speaker. None of these constructions sound particularly felicitous to me and I'm not finding any examples in a search of Irish-language corpora.

Regardless, de causes lenition and drops the e before a word that begins with a vowel sound. Since fh is silent, that means de + fine = d'fhine.