This is taken from the story Páidín Ó Ceallaigh agus an Easóg (written down by Douglas Hyde)

Bhí dúil aige le dul go h-aonach Cháthair-na-mart le storc asail do dhíol.

He translates this sentence as - He wanted to go to the fair of Cauher-na-mart to sell a 'sturk' of an ass that he had.

I have the impression that it means that his ass isn't worth very much ?

It's not a word I'm familiar with, but the dictionary gives this definition.

https://www.teanglann.ie/ga/fgb/Stirk

It seems to apply to cattle but I suppose it can apply to donkeys too. A yearling.

I should have looked it up in the Dinneen Dictionary (as someone has just told me to) - it's on Internet Archive and you can download it if you want to.

Storc - a large animal, a bullock - so here it would mean a large (strong) donkey, and probably male as well. Seo dhuit