St Barnabas was demolished many years ago - civil registratration started in 1877/8 thus you only have baptismal records - around 1911 these were transcribed by the Manx Registry (then known as Rolls office) and it is these entries that form the basis of any certificate - the mormons filmed the official copy and thus you can see the info by borrowing the 'parish register' film via your local FHC - at that period unlikely to give you any more than is in the IGI. You may find more info in the BMD's section of the Manx press (on microfilm at Manx Museum) as it occured in Douglas
(see manxnote linked to at top of page)