Short answer? Probably not.
Long answer? Well, let's put it like this. A lot of the Internet chatter about Michael Fassbender taking on one of the most recognisable characters in Irish early history and mythology stems from an extremely shady looking blog called 'Hollywood Spy', which reported back in January of this year that Fassbender - hot off the heels of Assassin's Creed - wanted to get his next big franchise going.
The film, according to Hollywood Spy's reporting, is going to be called Irish Myths and begin with the story of Cu Chulainn and Fassbender playing the lead. Like so much fake news, there's a kernel of truth in there somewhere.
It's true, Michael Fassbender was working on a film about Setanta, who became Cu Chulainn and so on, but it all went quiet in 2012. Fassbender was developing the project with a script by Ronan Bennett, a screenwriter from Northern Ireland who also worked with Michael Mann on Public Enemies. IMDb lists Bennett's latest project as Gunpowder, a TV miniseries about the November 5th Gunpowder Plot in English history.
For his part, Michael Fassbender has never mentioned Cu Chulainn in any interview he's done recently, and he's been doing quite a few for Alien: Covenant, and it's very possible that he was asked about future projects.
Considering that the report that started this all off is four months old and wasn't picked up or investigated by any of the trade journals - or anyone else for that matter - leads us to believe that this story is basically a fairytale straight from the annals of the Ulster Cycle.
It's a shame, because there's tons of material there that could conceivably turned into a great film and with a real budget behind it, it could really work.