How do you say little black dog in Gaelic.?

There isn't one single Gaelic language. There are several, including Irish Gaelic (Gaeilge) and Scottish Gaelic (Gàidhlig). The translation will differ slightly between them.

* Irish Gaelic (Gaeilge): A good translation would be madra beag dubh. (madra = dog, beag = little/small, dubh = black)

* Scottish Gaelic (Gàidhlig): A good translation would be cu beag dubh. (cu = dog, beag = little/small, dubh = black)

Both phrases are grammatically correct and understandable. The difference lies in the word for "dog."