http://www.ee.surrey.ac.uk/Contrib/manx/parishes/rcath/rc.htm for a description.
Brian Lawson's site has index to RC baptisms/marriages pre 1847 - the gap from there to 1877 (civil reg startes) is covered in data given to manx Museum and IoM FHS libraries by parish priest (but not published on Web). Prior to 1820's some Manx RC baptisms etc were recorded at Whitehaven which ran the manx mission (a list is on a notice board within St Mary's)