How do you say dark horse in Irish?

There isn't a single, direct translation for "dark horse" in Irish that captures the idiomatic meaning of an unexpected winner or contender. Irish would likely use a descriptive phrase instead. Here are a few options, depending on the nuance you want to convey:

* Capall dorcha: This is a literal translation, "dark horse," but it doesn't carry the idiomatic meaning. It would simply mean a horse that is dark in color.

* Duine nach raibh aon duine ag súil leis: This translates to "a person nobody was expecting," which gets closer to the meaning.

* Iontas mór: This means "big surprise," focusing on the unexpected nature of the dark horse's success.

* Ritheoir nach raibh aon duine ag súil leis a bheith rathúil: This translates to "a runner nobody expected to be successful," and is suitable if the context is a competition.

The best option will depend heavily on the context. For most situations, `duine nach raibh aon duine ag súil leis` (a person nobody was expecting) or `iontas mór` (big surprise) would be the most natural and accurate ways to convey the spirit of "dark horse" in Irish.