The word I learnt for this is something like boo-yawn bwee. Of course the last one is buidhe (yellow), but does anyone know how the first one is supposed to be spelt? I cant find anything online so it could be extremely dialectal.

Dineen has entries for the following.

Buidheán [note: not 'búidheán, as might be suggested by your spelling]: a) Yellow-hammer [type of bird'] b) yoke of egg.

Buidheán buidhe (Connacht word), pronounced 'budhan buidhe': a) a kind of flower [what kind of flower isn't specified) b) identical to 'breoghain' [but strangely, there is no entry under 'breoghain']

Buidheán óir: goldfinch

Buidheog (also baidheog): yellow-hammer [type of bird]

For 'golden-rod', teanglann.ie has 'slat óir'.

Teanglann.ie has the reformed spellings 'buíán' and 'buíóg'. Under 'Buíán buí', it has 'corn marigold' [type of flower].

Thank you for your response! Buidheán buidhe might be related... funny though its a Connacht word. My grandma who had this word was from the northern Déise area. Maybe the meaning was changed somewhere cross-dialectally, and some odd fada was added? Or maybe she just got her names mixed up. 'slat óir' to me doesn't sound very legitimate, like 'banc sneachta' its just a literal translation.

Buachalán Buí

Fairies would ride them on Halloween.

_________________
The dialect I use is Cork Irish.
Ar sgáth a chéile a mhairid na daoine, lag agus láidir, uasal is íseal

Thank you. That is probably it. She had a very soft almost 'h' broad ch sound and a y-like broad 'l' sound, which probably made it sound like boo-yawn.