The RC baptisms were derived from an 1847 submission (required by law) of all 'dissenter' (RC were not legally dissenters) baptisms and marriages - the original book is still at the registry - you can borrow the film at any FHC - a copy of the film (RB512 - Manx Museum number not the LDS no.) is in Manx Museum. Catholic records cover all Island but some additional notes in the register sometimes give a location (eg lives near Peel). Catholics had no burial ground of their own but sometimes buried their dead at Langness (see St Michaels island page on manxnotebook, sometimes at Cathedral on St Patrick's Island but usually in the C of E graveyards as these were the only ones on the Island (apart from one small Methodist one) - eg one Catholic prist is buried near the entrance to Old Kirk Braddan.
Castletown falls within the parish of Malew - the actual church is about 2km away - hence the use of the Chapel of Ease of St Mary's in middle of Castletown for baptisms (marriages were still mostly at malew) which had no burial yard.