John:
The NLI microfilmed parish registers in the 1950s and 1960s. Here is an excerpt from their site:
"The NLI microfilmed registers from the majority of Catholic parishes in Ireland and Northern Ireland during the 1950s and 1960s. Digital images from these microfilms are now freely available on the website: Catholic Parish Registers at the NLI."
They microfilmed what was available at the time and stopped at 1880 for privacy reasons. Later some parishes found an additional register but that info was not passed to NLI. Later when Roots Ireland or www.irishgenealogy.ie accessed local records they were given more records than what was provided to NLI.
I think that is the likely explanation of what you experienced with Castleisland records.
Roger McDonnell