I know of a couple rivers, and some of the best boaters in the country (it's a small country). There is the Flesk, a class 4 stretch that is about 25min to run without scouting. There is also the Gaddah (sp?) but that one is a class 5 I think, both are in Kerry county, not too far from each other.
When are you going? There is only water when it rains, no mountains so no snow melt... If you want I can give you a couple emails of those kayakers out there. Also check irishfreestyle.com (or .ie, not sure which)