as I said earlier for much of the 19th C the main RC church was St Marys (see www.manxnotebook.com for the earlier church of St Francis Xavier) - the church in Douglas provided priests for services etc at the other RC chapels though in early 20th Ramsey got its own priest - not sure re Castletown - Peel only got its own priest in 1930.
Thus there was only the one register kept (at least pre 1849 + prob for rest of 19th C - in the early reg which I have photos of some of the baptisms are noted as of Peel .
St Mary's would lie in the CofE parish of St Georges tho until mid 19th C St Georges was legally a chapel of ease to Braddan tho pre incorporation of Douglas as a borough it lay in the civil parish of Onchan (Manx affairs are never simple!) - however I suspect the reason for RC records coming under St Georges is simpler in that I suspect the batch of photographed registers commenced with St Georges and then included St Marys