Why is partition evident in parish records?

The National Library website ‘about’ section on the availability of the Catholic parish records states:

“The registers contain records of baptisms and marriages from the majority of Catholic parishes in Ireland and Northern Ireland up to 1880”.

I have always thought of Ireland as one island. Can I have been mistaken?

Surely the National Library of Ireland should know better than to have such a partitionist attitude, particularly bearing in mind that the Catholic Church in the whole island is and has always been all Ireland body.

Brian P O Cinneide

Essenwood Road

Durban

South Africa