So you think that "most" doesn't need qualification, but "some" does? Why not "most are there but some have to get up in the morning?"
Because "most" does need qualification - do you mean "most of them are there", (which is a bit of a non-sequitur after "I thought that you were all going to the party", which isn't talking about "them"), or "most of you" or "most of us", as "us" can include "you", but doesn't necessarily do so.
The Irish for "the majority of them are there" is the fairly straight forward "tá an formhór acu ansin" (or ann, if you prefer).
Both "a bhformhór" and "an formhór acu" are valid constructions, but it's pretty clear that "a bhformhór" isn't a "literal translation" of "the majority of them", and the lack of a definite article in the Irish sentence, even though there is a valid construction that uses the definite article, suggests that "most of them" is indeed, a better choice for the translation, as well as being how things are said in English (even though there isn't much to choose between them as far as meaning goes).
The focloir.ie entry for "most" includes "a bhformór" as one of translations for "most of them".
I accept that the original does not contain the definite article, which leaves me with "A majority of them are (or is) there" ("Their majority is there"), which all starts to get a bit clunky, at least in English. So most it is! Thank you both for helping me to work through to the more sensible translation.
As explained in the earlier comments, a bhformhór does actually mean "their majority", but "their majority is there" is not colloquial English - you would say "most of them are there" (and more to the point, if you wanted to say "most of them are there" in Irish, you'd say tá a bhformhór ann).
This sentence doesn't translate as "the majority of them are there, because that's tá an formhór acu ann.
I have a little doubt about translating this to English: would a native get the same meaning from “they’re here for the most part” or is this saying something like “the people are all there but parts of them are missing/elsewhere”, as in war survivors or inexperienced Hogwarts students? ^^’
A native English speaker would interpret "Most of them are here" as, for example, 10 out of 12 people are present. "They're here for the most part" conveys the same meaning, and it can also be interpreted as their mental presence; for example, if there are 12 people physically present, maybe only 10 of them are listening and paying attention (2 others are daydreaming). Both of these sentences, I suppose, could also be interpreted to mean what you propose, although it is NOT the first interpretation that comes to mind.