This is news to me; we've seen the film's first trailer, but she definitely wasn't in it. And even more interesting is that the project isn't listed on her IMDB page.
She was hand-picked by Hollywood heartthrob Brad Pitt to star in zombie film World War Z, and has described him as a 'lion' on set [...] Pitt was so impressed with Negga's performance [in Steve McQueen's Twelve Years A Slave, which they're both in] that he picked her for one of the roles in World War Z. Pitt obviously rates Negga's acting skills pretty highly [...] She was approached to do the film by his production company Plan B. "I don't know what he must have thought of me. To get the chance to be on such a big film, it was very daunting..."
Etc, etc, etc...
As for what character she plays In the film, which is out this summer, the profile only states that Ruth plays a scientist helping Pitt's character save the world.
Like I said, it's strange that the project is not listed on her IMDB page, and I couldn't confirm that there ever was an official announcement for it. And she's nowhere to be found in the trailer that was released in November.
But I reached out to Plan B to add more to the story, but no response yet.
The Ethiopian-Irish actress currently can be seen in the Channel 4's (UK) political thriller series, Secret State, playing a meddlesome MI6 agent.