As I might have mentioned before, there are two bilingual editions of this story - in Scots Gaelic and in Irish (available from Litríoch) as well as in Scots Gaelic and Scots (available from www.feisean.org). Most of the other versions that you can find in the Scots Gaelic oral tradition from Tobar an Dualchais/Kist o' Riches or from Cape Breton for example have the two characters reversed - where Biorachán Mór does all the searching while it's Biorachán Beag who ends up eating too many nuts.
If you get the itch to listen to it in Scots Gaelic, Meanbh Chuileag can oblige - and you can download it as well.

www.gaelic4parents.com/nip/read-aloud/b ... h-chuileag

In that case, it's contextual. When it's growing in the ground, as in a pasture, on a lawn, or in the wild it's grass. When it's being harvested (or being grown for harvest) or fed to animals, it's hay. You put your cow out to eat the grass, but you cut the hay and toss hay into her manger. You're growing hay in the south field, but you have grass on your lawn.

Redwolf