re RC Parishes :
St. Pauls - North city, the church is at Arran Quay
St. Catherine's - south city, Meath Street
St. Michan's - north city, North Anne Street/Halston St
St. Nicolas Without - south city, Francis Street
There's no direct correlation between the census returns and the RC parishes, you have to estimate the parish based the locations, or by checking other records for the same area (e.g. on IrishGenealogy).
You have to visit the National Library to view the microfilms, but many RC parish records for Dublin City are already available online on
www.irishgenealogy.ie , more are due to be added later in the year.
Shane