"The Myth" is not quite as monolithic as all that- there are a couple of different recensions of the Tain, and lots of little individual parts that are traditionally grouped with it. Kinsella's translation, although a true masterpiece, went a long way towards enshrining the notion that there is one and only one Tain, which is not quite the case.

Also, let's not forget that the Tain is not history, nor is it really the end result of a long oral tradition, so don't look it for accurate information about Late Iron Age Ireland. Barry Raftery is the man for that (Pagan Celtic Ireland).

Which is why, I suppose, people feel free to do their own versions of the Tain in film and comic. There's lots in Kinsella's version that wouldn't translate well to the screen- although, the version that Genava linked to looks like a truly epic piece of crap. It's a shame, as I think that there is a really good Cuchullain movie that could be made.