So when you are searching for your ancestors especially in Ireland, you will have places called registration districts but what you'll always have to remember is that the place that you are looking for might not be a registration district, but don't get disheartened! How I figure it out is I punch in the place I think that the person was born (e.g. Ring in Co. Waterford) into Google Maps and I go looking for the distance from Ring to each of the districts. So in this case Ring is closest to Dungarvan so then I'd go looking for a cert of that person in Dungarvan. But you're probably going to have to find registration districts all by yourself won't you?
No you won't! I have all of them summed up in a 2 page document in this post. civil_registration_districts.docx click on file name highlighted in bold text to the left.