Translation:Is beag eatarthu.
I’m not sure — both seem to be used. The English-Irish Dictionary provides as examples both Is beag eatarthu, translated there as “There is not much difference between them”, and Tá easaontas eatarthu, translated there as “There is friction between them”.
For this sentence, I would have expected beagán to be used in the translation rather than beag.
EDIT: Perhaps is is used when an “equality” comparison is involved (e.g. ”little between them“ suggesting that they’re quite similar), and bí is used when such a comparison is not involved (e.g. describing the existence of the abstract noun “friction” between them).
Maybe it could be considered elliptical ie "Is beag atá eatarthu" "It's a small amount there is between them", so to speak.
Sometimes an a bheith seems to be optional, but I didn’t know of an atá in the same boat.
I can't attest to that, myself, but it seemed like a possible explanation.
I have just found "Is beag atá eatarthu" listed in a book on Google (Impreasin na Gaeilge by Seosamh Mac Ionnrachtaigh) with the translation "There is little between them".
You can say "Is beag X" meaning "X is a little thing"
Is beag an difear atá eadarthu = The difference between them is a small thing = They are similar
However just saying "eadarthu" without a noun attached to it seems unnatural to me, I think this sentence is slight Béarlachas.