I'd imagine these texts are in middle or even old irish as opposed to the modern spoken version. Be a good place to start though. Some of the more well known texts are available online at http://www.ucc.ie/celt/publishd.html while the some of the original manuscripts can be seen at http://www.isos.dias.ie/english/index.html
You'd really need to be a scholar to make head or tail of them though, I've 12 years of school irish and they might as well be latin.