It's great that you are considering touring Ireland by campervan!
There are many campsites and caravan parks around the island to suit your needs. More often than not, they also have facilities to empty and refill your water tanks. Most of them can be found on the Camping Ireland website
, which also includes links to campervan rental companies.
We also have a page
about camping in Ireland as well as this previous thread
that you might find useful.
Do you already have an itinerary in mind?