I'm trying to say - that's the eldest sister of the three of us.
Here's my go at it -
Is í súd is sine as (or ó ?) triúr eadrainn (or dúinn ?)

Here's my go at it:

Is í siúd an driothúr† is sine den triúr againn. (GC)

Await further input ...


†driofúr i nGaeilge na Mumhan, b'fhéidir?

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Múinteoir Gaeilge - Irish Teacher
My "specialty" is Connemara Irish, particularly Cois Fhairrge dialect, but I can also speak Ulster and Munster Irish with native-level pronunciation.
Is fearr Gaeilge ḃriste ná Béarla cliste, cinnte, aċ i ḃfad níos fearr aríst í Gaeilge ḃinn ḃeo na nGaeltaċtaí.
Gaeilge Chonnacht (GC), go háraid Gaeilge Chois Fhairrge (GCF), Gaeilic Uladh (GU), Gaelainn na Mumhan (GM), agus Gaeilge an Chaighdeáin Oifigiúil (CO).

I forgot to put in the word for sister - in my book it says deirfiúr - but the pronunciation is dr' (upside down e) ' fu:r - but they tend to give a standard spelling along with a Munster pronunciation which doesn't really seem to coincide. For example Irish is written Gaeilge but pronounced approximately as Gaylinye, so there's probably a different way of writing sister in Munster Irish.

Yes in the Cork Irish wordpress dictionary (I quote) it is - "dríofúr f. (gs. dríféar, ds. drifír, npl. driféaracha, gpl. driféar) as well as after collective numerals the genetive plural is driféar: an bheirt driféar - drifiúr is a variant also found in Cork literature."
I'm never quite sure which preposition is needed for - in this case - of us.

normally if it's "of us, of yous, of them" it is "againn, agaibh, acu".
You use "de/do" before nouns: ceathrar de/do na daoiní sin, etc

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Is fearr Gaeilg na Gaeltaċta ná Gaeilg ar biṫ eile
Agus is í Gaeilg Ġaoṫ Doḃair is binne

There's also

Tá sí ar an deirfúir is sine…..
She is the oldest sister....

Here's a bit more context -

And she said "Roll this before you
and it will take you to another sister of mine -
that's the eldest sister of the three of us,
srl...