Ireland is okay for hitchhiking. The recent economic upsurge might have changed this situation a little bit though.
Hithchiking is the easiest and fastest way to travel in the countryside and in the less inhabitated regions, where public transportation such as train or bus is not available or it is not as frequent as a traveller might expect (one or two buses per day). You can easily get multiple lifts from one village to another for really short distances, and people are very welcoming.
It might be therefore more difficult to get lifts along main national roads such as the Ennis-Galway going out from Ennis, where traffic is heavy and the hitchhiking spots are not the best ones (no spaces along the road for letting cars stop).