Hello Jack
According to the official website (
www.walkireland.ie): "For the most part, camping is only permitted in official sites. In many cases though, local landowners don't mind people camping as long as permission is sought in advance, due care is taken and the country code is respected."
Though I haven't camped along the Way myself, I think you'd need to be fully prepared for coping with dodgy water supplies (sheep) and difficulties in finding someone to ask for permission, not to mention discreet sites.
In any event, if you're keen, why not give it a go, but be prepared to retreat to commercial campsites, where they're available.
Enjoy the walk!
Best wishes
Sandra