There is a gap in Roman Catholic Records from 1849 to 1877 - those from 1814 to 49 were copied and given in (now indexed on Brian Lawson's site) when the law changed re allowing marriage/baptism in non-Establishment chapels (in effect retrospectively accepting the validity of these marriages - the independents and presbyterians actually got married off Island or in the Established church but the Catholics would not usually acknowledge the need )
Post 1849 such marriages/baptism were ok but it appears not entered into the civil records - hence the need to search the RC register; RC records per se are generally not given to the Mormons (IGI) for religious reasons - all RC records on Mormon files are there because the records needed to be given to some civil authority from whom they were then obtained

One of the priests at St Mary's was interested in genealogy and had 'promised' at some time to transcribe any register for use by others but nothing appears to have come of this