You should be able to get the 1841 census of Douglas on Ancestry UK.
However Brian's census index has only:
Surname Forename Age Parish Piece Folio Page
FAGAN James 27 Dou 1465/8 30 18
See:
www.iomfhs.im/lawsons/census/c41-f.html

The reason I wrote that you need to read Brian's introduction to the Dissenter bapts/births was because he wrote "Perusal of the original records is highly recommended as much additional information is given...", and also gave the LDS film number (LDS film 0106721) for the records he indexed. He wouldn't still have these records because the film would have been returned.

The LDS catalog has:
Registry of baptisms and marriages of the Catholics of the Isle of Man, 1817-1849 FHL BRITISH Film 106721 Item 2 (you have the entry numbers from Brian's index). These films can be ordered to view through any LDS Family History Centre. See under Church of Jesus Christ of Latter Day Saints in the phone book.
You may also find their marriage on the same film.

Sue