The fact that 'idir' can mean 'both' came up on Scooby's réamhfhocail lesson thread recently: bhí idir bhuachaillí agus chailíní sa rang.

Bhí mé ar an Luas inniu (great place to hear/read a few placenames i nGaeilge agus Béarla) agus léigh mé an rud seo ar an ardán:

Féach gach treo / Look both ways.

My question is this: Why not 'Féach idir treo' (h?) or does that work in this case?

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"idir" (Ulster eadar) only means "both" when it's followed by two words linked with "agus/is":

eadar aosta agus óg
eadar fhir agus mhná...

otherwise, it means "between" (and it doesn't lenite). "féach idir treo" would mean "look between direction", which doesn't make sense.

Btw I would say "Féach ar gach treo", or "amhanc ar an dá thaobh"

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

Duly noted under "idir."

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Táim ag foghlaim Gaelainn na Mumhan

Tá fáilte roim nach aon cheartú!
I am a learner. Any translations offered are practice and should not be used unless confirmed.

As we say in Conamara -

Breathnaigh 'chaon taobh.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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As you can see from the posts, there are various valid and tried and tested ways of putting this.
To me 'Féach gach treo' is easily understood - I would not use 'idir'.

Just to add to the mix, you could if you wished also translate it as:
Féach sa dá threo
Breathnaigh sa dá threo
(fixed the 'sa dhá')
... but I think you'd get away with it as it is.

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

sa dhá or sa dá?
the lenition looks strange to me after the article included in "sa" (but maybe you say it in Connemara...)

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

If I was saying it I would say it
san dá threo
so I suppose I should have written it so
... there I was making up my own language again LOL
grma for that.
...must look up that new CO regarding sa dhá. LOL.
Can you write it?

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Bí cinnte de go nglacfaidh triúr le gach aistriúchán a thabharfar.
Be sure to get three in agreement with a translation given.

In the CO i think it's "sa dá..." because normally you can't have "dhá" after the article, "sa dá" is "ins an dá", of course. That's why I was surprised

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

I agree.

I found this particular use of 'idir' rather difficult to get my head round, as I was trying to understand it through the English translation, I suppose.
Her name was Gráinne, but she wasn't at all like the princess in the old tale. (no trace of 'between' there)
Gráinne an t-ainm a bhí uirthi, ach ní raibh pioc cosúlachta idir í agus an banfhlaith sa sean-scéal.
Admittedly there's this sentence on Pota Focal - Tá an-chuid cosúlachtaí idir an dá rud - There are many similarities between the two things, but the use of idir in the first sentence does seem - to me at least - to be very idiomatic.