the Roman Catholic Archdiocese of Dublin says that it “does not make use of baptismal registers for calculating the Catholic population of the Archdiocese of Dublin. It relies solely on the data from the Central Statistics Office, obtained through the census, by which citizens themselves choose to record, or not, their religious affiliation.”These numbers feed into decisions on funding for schools for example and if they can be used by the Catholic church to defend their control of most of our schools, they will be, even if the church knows they do not represent the number of genuinely devout Catholics (if we use regular mass attendence as a criteria).
If the census asked a less leading question than "What is your religion?", such as "How religious are you?", then we might get a better picture. Unfortunately Atheist Ireland was unable to get the question changed for the 2011 census. I think the census results will be published in July and it'll be interesting reading. However it is unlikely to be an accurate reading, as far as religion is concerned.