Could this also mean we have everything as in everything we need is to hand, is lying around, is in our vicinity?
You're right, technically speaking, the sense of "nothing" comes from the negative participle of the associated verb: níl rud ar bith ag tarlú would usually be translated as "there is nothing happening" but An bhfuil rud ar bith ag tarlú? could be "Is anything happening?", though An bhfuil aon rud ag tarlú? might be the more usual way of asking that question. anything".
But if you answer the question Cad atá uait? with Rud ar bith, you are saying that you "want nothing", not that you "want anything".
You're right, 'ar bith' seems more prominent in Ulster. One thing I've noticed, with the spoken language anyway, is that questions can be answered in the negative using ar bith. eg. Q. An bhfuil rud ar bith le déanamh agat? (Do you have anything to do?) A. Rud ar bith. (No) Q. An bhfuil airgead ar bith agat? (Do you have any money?) A. Airgead ar bith. (No) Anyone else noticed this?
That's probably not how you'd say "All about us" in Irish, even though it is a rough literal translation (gach rud is more literally "every thing").
There are a number of examples of sentences in the NEID that use "all about" in the same sense as this phrase, but they use different idioms in Irish:
"that's what it's all about" - sin a bhfuil i gceist leis dáiríre, sin é is bun agus barr leis
"that's what the story is all about" - sin é bun agus barr an scéil
"it's all about getting the figures right" - na figiúirí a bheith i gceart an rud is tábhachtaí
"Christmas is all about commercialism" -níl i gceist leis an Nollaig ach airgead a dhéanamh
"it's all about bums on seats for her" - is cuma léi siúd ach pinginí a dhéanamh
"it's all about the advancement of her own career" - níl i gceist ach a gairmréim féin a chur chun tosaigh
I think that i gceist linn ar fad has a nice ring to it!
I read "all about us" in your song title as meaning "it is completely about us" or "nothing else matters", so ar fad makes more sense than gach rud. i gceist le literally means "in question with", but means "about" in the sense of "concerning", and it goes well with ar fad.
In the other examples, bun agus barr is literally "bottom and top" and expresses totality/completeness, airgead/pinginí a dheanamh is "making money(pennies)". Is cuma is complicated, and I'm not going into it here :-)
It's also worth nothing that, while it's best to be fairly literal when learning, and doing exercises in Duolingo, in the real world the best translation is often quite different and the NEID often presents a number of different ways to say the same thing, so "it's all about getting the figures right" is translated as na figiúirí a bheith i gceart an rud is tábhachtaí which "back-translates" as "the figures being right is the most important thing".
Thanks this really helped! Irish is very different from all the languages I've learnt so far so I'm startled and confused all the time when I find new features but I feel like I am slowly starting to get the knack of it or at least some bits of it XD
I did figure it probably wasn't really how you said it but I have found that connecting Duolingo sentences to songs or other pop references really helps me remember new words and forms.
But thank you so much for taking the time to tell me how it's really expressed in Irish and for even breaking it down for me. I hope one day when I visit Ireland and its Gaeltachtaí I'll be able to at least partially understand and make myself understood without having to resort to English :)