Just so I'm clear, Tá ribe gearr gruaige ar mo dheartháir would be "There's a short hair on my brother". Correct?
How would you phrase the statement if my brother were bald (no hair)?
I suppose you could say Níl aon gruaig ar mo dheartháir.
Which means "My brother doesn't have any hair".
According to teanglann.ie, maol is the word for “bald”.
Gan ghruaig is the term they use for “without hair”.